Method and apparatus for transmitting a signal within a predetermined spectral mask

ABSTRACT

An apparatus and method are described for transmitting a signal within a predetermined spectral mask defining a range of power density limits across a frequency range. A signal source provides an input signal to a low-pass filter, which filters the input signal to produce a filtered signal. The filter is selectively operable in a first mode in which the input signal is filtered within a narrower bandwidth, and in a second mode in which the input signal is filtered within a broader bandwidth. The filtered signal is output to a modulator, which modulates the filtered signal to produce a modulated signal. The modulated signal is output to a transmitter, which transmits the modulated signal.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present Application for Patent claims priority to ProvisionalApplication No. 61/151,497, entitled “Method and Apparatus forTransmitting a Signal Within a Predetermined Spectral Mask” filed Feb.10, 2009; Provisional Application No. 61/153,285, entitled “Method andApparatus for Transmitting a Signal Withing a Predetermined SpectralMask” filed Feb. 17, 2009; and Provisional Application No. 61/240,012,entitled “Method of Adapting Pulse Shape of Data Pulses For TransmissionIn a Digital Communications System” filed Sep. 4, 2009 and herebyexpressly incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates generally to the field of radio communications andin particular to the increasing of channel capacity in a radiocommunications system.

BACKGROUND

More and more people are using mobile communication devices, such as,for example, mobile phones, not only for voice but also for datacommunications. In the GSM/EDGE Radio Access Network (GERAN)specification, GPRS and EGPRS provide data services. The standards forGERAN are maintained by the 3GPP (Third Generation Partnership Project).GERAN is a part of Global System for Mobile Communications (GSM). Morespecifically, GERAN is the radio part of GSM/EDGE together with thenetwork that joins the base stations (the Ater and Abis interfaces) andthe base station controllers (A interfaces, etc.). GERAN represents thecore of a GSM network. It routes phone calls and packet data from and tothe PSTN and Internet and to and from remote stations, including mobilestations. UMTS (Universal Mobile Telecommunications System) standardshave been adopted in GSM systems, for third-generation communicationsystems employing larger bandwidths and higher data rates. GERAN is alsoa part of combined UMTS/GSM networks.

The following issues are present in today's networks. First, moretraffic channels are needed which is a capacity issue. Since there is ahigher demand of data throughput on the downlink (DL) than on the uplink(UL), the DL and UL usages are not symmetrical. For example a mobilestation (MS) doing FTP transfer is likely to be given 4D1U, which couldmean that it takes four users resources for full rate, and eight usersresources for half rate. As it stands at the moment, the network has tomake a decision whether to provide service to 4 or 8 callers on voice or1 data call. More resources will be necessary to enable DTM (dualtransfer mode) where both data calls and voice calls are made at thesame time.

Second, if a network serves a data call while many new users also wantvoice calls, the new users will not get service unless both UL and DLresources are available. Therefore some UL resource could be wasted. Onthe one hand, there are customers waiting to make calls and no servicecan be made; on the other hand, the UL is available but wasted due tolack of pairing DL.

Third, there is less time for mobile stations (also known as UserEquipment or UE) working in multi-timeslot mode to scan neighbor cellsand monitor them, which may cause call drops and performance issues.

FIG. 1 shows a block diagram of a transmitter 118 and a receiver 150 ina wireless communication system. For the downlink, the transmitter 118may be part of a base station, and receiver 150 may be part of awireless device (remote station). For the uplink, the transmitter 118may be part of a wireless device, and receiver 150 may be part of a basestation. A base station is generally a fixed station that communicateswith the wireless devices and may also be referred to as a Node B, anevolved Node B (eNode B), an access point, etc. A wireless device may bestationary or mobile and may also be referred to as a remote station, amobile station, user equipment, mobile equipment, a terminal, a remoteterminal, an access terminal, a station, etc. A wireless device may be acellular phone, a personal digital assistant (PDA), a wireless modem, awireless communication device, a handheld device, a subscriber unit, alaptop computer, etc.

At transmitter 118, a transmit (TX) data processor 120 receives andprocesses (e.g., formats, encodes, and interleaves) data and providescoded data. A modulator 130 performs modulation on the coded data andprovides a modulated signal. Modulator 130 may perform Gaussian minimumshift keying (GMSK) for GSM, 8-ary phase shift keying (8-PSK) forEnhanced Data rates for Global Evolution (EDGE), etc. GMSK is acontinuous phase modulation protocol whereas 8-PSK is a digitalmodulation protocol. A transmitter unit (TMTR) 132 conditions (e.g.,filters, amplifies, and upconverts) the modulated signal and generatesan RF modulated signal, which is transmitted via an antenna 134.

At receiver 150, an antenna 152 receives RF modulated signals fromtransmitter 110 and other transmitters. Antenna 152 provides a receivedRF signal to a receiver unit (RCVR) 154. Receiver unit 154 conditions(e.g., filters, amplifies, and downconverts) the received RF signal,digitizes the conditioned signal, and provides samples. A demodulator160 processes the samples as described below and provides demodulateddata. A receive (RX) data processor 170 processes (e.g., deinterleavesand decodes) the demodulated data and provides decoded data. In general,the processing by demodulator 160 and RX data processor 170 iscomplementary to the processing by modulator 130 and TX data processor120, respectively, at transmitter 110.

Controllers/processors 140 and 180 direct operation at transmitter 118and receiver 150, respectively. Memories 142 and 182 store program codesin the form of computer software and data used by transmitter 118 andreceiver 150, respectively.

FIG. 2 shows a block diagram of a receiver unit 154 and demodulator 160at receiver 150 in FIG. 1. Within receiver unit 154, a receive chain 440processes the received RF signal and provides I and Q baseband signals,which are denoted as I_(bb) and Q_(bb). Receive chain 440 may performlow noise amplification, analog filtering, quadrature downconversion,etc. An analog-to-digital converter (ADC) 442 digitalizes the I and Qbaseband signals at a sampling rate of f_(adc) and provides I and Qsamples, which are denoted as I_(adc) and Q_(adc). In general, the ADCsampling rate f_(adc) may be related to the symbol rate f_(sym) by anyinteger or non-integer factor.

Within demodulator 160, a pre-processor 420 performs pre-processing onthe I and Q samples from ADC 442. For example, pre-processor 420 mayremove direct current (DC) offset, remove frequency offset, etc. Aninput filter 422 filters the samples from pre-processor 420 based on aparticular frequency response and provides input I and Q samples, whichare denoted as I_(in) and Q_(in). Filter 422 may filter the I and Qsamples to suppress images resulting from the sampling by ADC 442 aswell as jammers. Filter 422 may also perform sample rate conversion,e.g., from 24× oversampling down to 2× oversampling. A data filter 424filters the input I and Q samples from input filter 422 based on anotherfrequency response and provides output I and Q samples, which aredenoted as I_(out) and Q_(out). Filters 422 and 424 may be implementedwith finite impulse response (FIR) filters, infinite impulse response(IIR) filters, or filters of other types. The frequency responses offilters 422 and 424 may be selected to achieve good performance. In onedesign, the frequency response of filter 422 is fixed, and the frequencyresponse of filter 424 is configurable.

An adjacent channel interference (ACI) detector 430 receives the input Iand Q samples from filter 422, detects for ACI in the received RFsignal, and provides an ACI indicator to filter 424. The ACI indicatormay indicates whether or not ACI is present and, if present, whether theACI is due to the higher RF channel centered at +200 KHz and/or thelower RF channel centered at −200 KHz. The frequency response of filter424 may be adjusted based on the ACI indicator, as described below, toachieve good performance.

An equalizer/detector 426 receives the output I and Q samples fromfilter 424 and performs equalization, matched filtering, detection,and/or other processing on these samples. For example,equalizer/detector 426 may implement a maximum likelihood sequenceestimator (MLSE) that determines a sequence of symbols that is mostlikely to have been transmitted given a sequence of I and Q samples anda channel estimate.

The Global System for Mobile Communications (GSM) is a widespreadstandard in cellular, wireless communication. GSM employs a combinationof Time Division Multiple Access (TDMA) and Frequency Division MultipleAccess (FDMA) for the purpose of sharing the spectrum resource. GSMnetworks typically operate in a number of frequency bands. For example,for uplink communication, GSM-900 commonly uses a radio spectrum in the890-915 MHz bands (Mobile Station to Base Transceiver Station). Fordownlink communication, GSM 900 uses 935-960 MHz bands (base station tomobile station). Furthermore, each frequency band is divided into 200kHz carrier frequencies providing 124 RF channels spaced at 200 kHz.GSM-1900 uses the 1850-1910 MHz bands for the uplink and 1930-1990 MHzbands for the downlink. Like GSM 900, FDMA divides the GSM-1900 spectrumfor both uplink and downlink into 200 kHz-wide carrier frequencies.Similarly, GSM-850 uses the 824-849 MHz bands for the uplink and 869-894MHz bands for the downlink, while GSM-1800 uses the 1710-1785 MHz bandsfor the uplink and 1805-1880 MHz bands for the downlink.

Each channel in GSM is identified by a specific absolute radio frequencychannel identified by an Absolute Radio Frequency Channel Number orARFCN. For example, ARFCN 1-124 are assigned to the channels of GSM 900,while ARFCN 512-810 are assigned to the channels of GSM 1900. Similarly,ARFCN 128-251 are assigned to the channels of GSM 850, while ARFCN512-885 are assigned to the channels of GSM 1800. Also, each basestation is assigned one or more carrier frequencies. Each carrierfrequency is divided into eight time slots (which are labeled as timeslots 0 through 7) using TDMA such that eight consecutive time slotsform one TDMA frame with a duration of 4.615 ms. A physical channeloccupies one time slot within a TDMA frame. Each active wirelessdevice/user is assigned one or more time slot indices for the durationof a call. User-specific data for each wireless device is sent in thetime slot(s) assigned to that wireless device and in TDMA frames usedfor the traffic channels.

Each time slot within a frame is used for transmitting a “burst” of datain GSM. Sometimes the terms time slot and burst may be usedinterchangeably. Each burst includes two tail fields, two data fields, atraining sequence (or midamble) field, and a guard period (GP). Thenumber of symbols in each field is shown inside the parentheses. A burstincludes 148 symbols for the tail, data, and midamble fields. No symbolsare sent in the guard period. TDMA frames of a particular carrierfrequency are numbered and formed in groups of 26 or 51 TDMA framescalled multi-frames.

FIG. 3 shows example frame and burst formats in GSM. The timeline fortransmission is divided into multiframes. For traffic channels used tosend user-specific data, each multiframe in this example includes 26TDMA frames, which are labeled as TDMA frames 0 through 25. The trafficchannels are sent in TDMA frames 0 through 11 and TDMA frames 13 through24 of each multiframe. A control channel is sent in TDMA frame 12. Nodata is sent in idle TDMA frame 25, which is used by the wirelessdevices to make measurements for neighbor base stations.

FIG. 4 shows an example spectrum in a GSM system. In this example, fiveRF modulated signals are transmitted on five RF channels that are spacedapart by 200 KHz. The RF channel of interest is shown with a centerfrequency of 0 Hz. The two adjacent RF channels have center frequenciesthat are +200 KHz and −200 KHz from the center frequency of the desiredRF channel. The next two nearest RF channels (which are referred to asblockers or non-adjacent RF channels) have center frequencies that are+400 KHz and −400 KHz from the center frequency of the desired RFchannel. There may be other RF channels in the spectrum, which are notshown in FIG. 3 for simplicity. In GSM, an RF modulated signal isgenerated with a symbol rate of f_(sym)=13000/40=270.8 kilosymbols/second (Ksps) and has a −3 dB bandwidth of up to ±135 KHz. TheRF modulated signals on adjacent RF channels may thus overlap oneanother at the edges, as shown in FIG. 4.

One or more modulation schemes are used in GSM to communicateinformation such as voice, data, and/or control information. Examples ofthe modulation schemes may include GMSK (Gaussian Minimum Shift Keying),M-ary QAM (Quadrature Amplitude Modulation) or M-ary PSK (Phase ShiftKeying), where M=2^(n), with n being the number of bits encoded within asymbol period for a specified modulation scheme. GMSK, is a constantenvelope binary modulation scheme allowing raw transmission at a maximumrate of 270.83 kilobits per second (Kbps).

GSM is efficient for standard voice services. However, high-fidelityaudio and data services desire higher data throughput rates due toincreased demand on capacity to transfer both voice and data services Toincrease capacity, the General Packet Radio Service (GPRS), EDGE(Enhanced Data rates for GSM Evolution) and UMTS (Universal MobileTelecommunications System) standards have been adopted in GSM systems.

General Packet Radio Service (GPRS) is a non-voice service. It allowsinformation to be sent and received across a mobile telephone network.It supplements Circuit Switched Data (CSD) and Short Message Service(SMS). GPRS employs the same modulation schemes as GSM. GPRS allows foran entire frame (all eight time slots) to be used by a single mobilestation at the same time. Thus, higher data throughput rates areachievable.

The EDGE standard uses both the GMSK modulation and 8-PSK modulation.Also, the modulation type can be changed from burst to burst. 8-PSKmodulation in EDGE is a linear, 8-level phase modulation with 3π/8rotation, while GMSK is a non-linear, Gaussian-pulse-shaped frequencymodulation. However, the specific GMSK modulation used in GSM can beapproximated with a linear modulation (i.e., 2-level phase modulationwith a π/2 rotation). The symbol pulse of the approximated GMSK and thesymbol pulse of 8-PSK are identical.

In GSM/EDGE, frequency bursts (FB) are sent regularly by the BaseStation (BS) to allow Mobile Stations (MS) to synchronize their LocalOscillator (LO) to the Base Station LO, using frequency offsetestimation and correction. These bursts comprise a single tone, whichcorresponds to an all “0” payload and training sequence. The all zeropayload of the frequency burst is a constant frequency signal, or asingle tone burst. When in power-on or camp-on mode or when firstaccessing the network, the remote station hunts continuously for afrequency burst from a list of carriers. Upon detecting a frequencyburst, the MS will estimate the frequency offset relative to its nominalfrequency, which is 67.7 KHz from the carrier. The MS LO will becorrected using this estimated frequency offset. In power-on mode, thefrequency offset can be as much as +/−19 KHz. The MS will periodicallywake up to monitor the frequency burst to maintain its synchronizationin standby mode. In the standby mode, the frequency offset is within ±2KHz.

Modern mobile cellular telephones are able to provide conventional voicecalls and data calls. The demand for both types of calls continues toincrease, placing increasing demands on network capacity. Networkoperators address this demand by increasing their capacity. This isachieved for example by dividing or adding cells and hence adding morebase stations, which increases hardware costs. It is desirable toincrease network capacity without unduly increasing hardware costs, inparticular to cope with unusually large peak demand during major eventssuch as an international football match or a major festival, in whichmany users or subscribers who are located within a small area wish toaccess the network at one time. When a first remote station is allocateda channel for communication (a channel comprising a channel frequencyand a time slot), a second remote station can only use the allocatedchannel after the first remote station has finished using the channel.Maximum cell capacity is reached when all the allocated channelfrequencies are used in the cell and all available time slots are eitherin use or allocated. This means that any additional remote station userwill not be able to get service. In reality, another capacity limitexists due to co-channel interferences (CCI) and adjacent channelinterferences (ACI) introduced by high frequency re-use pattern and highcapacity loading (such as 80% of timeslots and channel frequencies).

Network operators have addressed this problem in a number of ways, allof which require added resources and added cost. For example, oneapproach is to divide cells into sectors by using sectored, ordirectional, antenna arrays. Each sector can provide communications fora subset of remote stations within the cell and the interference betweenremote stations in different sectors is less than if the cell were notdivided into sectors and all the remote stations were in the same cell.Another approach is to divide cells into smaller cells, each new smallercell having a base station. Both these approaches are expensive toimplement due to added network equipment. In addition, adding cells ordividing cells into several smaller cells can result in remote stationswithin one cell experiencing more CCI and ACI interference fromneighboring cells because the distance between cells is reduced.

SUMMARY OF THE INVENTION

The invention is defined by the appended claims comprising: atransmitting apparatus; a method; a computer readable medium; and acomputer program product.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further features of the invention are set forthwithparticularity in the appended claims and together with advantagesthereof will become clearer from consideration of the following detaileddescription of exemplary embodiments of the invention given by way ofexample with reference to the accompanying drawings wherein:

FIG. 1 shows a block diagram of a transmitter and a receiver.

FIG. 2 shows a block diagram of a receiver unit and a demodulator.

FIG. 3 shows example frame and burst formats in GSM.

FIG. 4 shows an example spectrum in a GSM system.

FIG. 5 is a simplified representation of a cellular communicationssystem;

FIG. 6 shows an arrangement of cells in a cellular system;

FIG. 7 shows an example arrangement of time slots for a time divisionmultiple access (TDMA) communications system;

FIG. 8A shows an apparatus for operating in a multiple accesscommunication system to produce first and second signals sharing asingle channel;

FIG. 8B shows an apparatus for operating in a multiple accesscommunication system to produce first and second signals sharing asingle channel and using a combiner to combine first and secondmodulated signals;

FIG. 9 of the accompanying drawings is a flowchart of a method of usingthe apparatus shown in any of FIG. 8A or 8B;

FIG. 10A shows an example embodiment wherein the method described byFIG. 9 would reside in the base station controller;

FIG. 10B is a flowchart illustrating the steps executed by the basestation controller of FIG. 10A;

FIG. 11 shows the flow of signals in a base station;

FIG. 12 shows example arrangements for data storage within a memorysubsystem which might reside within a base station controller (BSC) of acellular communication system.

FIG. 13 shows an example receiver architecture for a remote stationhaving the DARP feature;

FIG. 14 shows part of a GSM system adapted to assign the same channel totwo remote stations;

FIG. 15 of the accompanying drawings illustrates a first example of anapparatus for combining and transmitting two signals with differentamplitudes;

FIG. 16 of the accompanying drawings illustrates a second example of anapparatus for combining and transmitting two signals with differentamplitudes;

FIG. 17 of the accompanying drawings illustrates a third example of anapparatus for combining and transmitting two signals with differentamplitudes;

FIG. 18 of the accompanying drawings illustrates a fourth example of anapparatus for combining and transmitting two signals with differentamplitudes;

FIG. 19 illustrates an apparatus for combining two signals by mappingtwo users' data onto the I and Q axis respectively of a QPSKconstellation;

FIG. 20 is a QPSK constellation diagram;

FIG. 21A of the accompanying drawings shows a flowchart illustrating thesteps for combining and transmitting two signals with differentamplitudes;

FIG. 21B of the accompanying drawings shows a flowchart illustrating thesteps for combining signals by mapping both users the I and Q axisrespectively of a QPSK constellation;

FIG. 21C of the accompanying drawings shows a flowchart illustrating thesteps for combining and transmitting two signals with differentamplitudes;

FIG. 22 is a flowchart comprising illustrating the steps taken whenadapting a non-MUROS base station to identify an enabledMUROS-capability in a remote base station;

FIG. 23 shows a base station with software stored in memory which mayexecute the method illustrated in FIGS. 21A, 21B, 21C and 22;

FIG. 24 is a diagram of a transmitting apparatus 240;

FIG. 25 is a diagram of a transmitting apparatus 250 for transmittingtwo signals substantially simultaneously;

FIG. 26 is a diagram of a transmitting apparatus 260 for transmittingtwo signals in combination;

FIG. 27 is a flow diagram representing a method for filtering,modulating and transmitting a signal according to a first or second modeof operation;

FIG. 28 is a diagram of a method for filtering, modulating, combiningand transmitting two signals according to a first and second mode ofoperation respectively;

FIG. 29 is a diagram of a transmitting apparatus 290 for combining andtransmitting two signals to produce a QPSK-modulated, phase-rotatedsignal;

FIG. 30 is a graph 300 showing a broader pulse 301 and a narrower pulse302;

FIG. 31 is a graph 310 showing test results;

FIG. 32 is a graph showing the performances 321 to 324 of fourreceivers; and

FIG. 33 comprises a selection of narrowed and broadened pulses.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of thepresent invention and is not intended to represent the only embodimentsin which the present invention can be practiced. The term “exemplary”used throughout this description means “serving as an example, instance,or illustration,” and should not necessarily be construed as preferredor advantageous over other embodiments. The detailed descriptionincludes specific details for the purpose of providing a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout these specific details. In some instances, well known structuresand devices are shown in block diagram form in order to avoid obscuringthe concepts of the present invention.

Interference due to other users limits the performance of wirelessnetworks. This interference can take the form of either interferencefrom neighboring cells on the same frequency, known as CCI, discussedabove, or neighboring frequencies on the same cell, known as ACI, alsodiscussed above.

Single-antenna interference cancellation (SAIC) is used to reduceCo-Channel Interference (CCI), The 3G Partnership Project (3GPP) hasstandardized SAIC performance. SAIC is a method used to combatinterference. The 3GPP adopted downlink advanced receiver performance(DARP) to describe the receiver that applies SAIC.

DARP increases network capacity by employing lower reuse factors.Furthermore, it suppresses interference at the same time. DARP operatesat the baseband part of a receiver of a remote station. It suppressesadjacent-channel and co-channel interference that differ from generalnoise. DARP is available in previously defined GSM standards (sinceRel-6 in 2004) as a release-independent feature, and is an integral partof Rel-6 and later specs. The following is a description of two DARPmethods.

The first DARP method is the joint detection/demodulation (JD) method.JD uses knowledge of the GSM signal structure in adjacent cells insynchronous mobile networks to demodulate one of several interferencesignals in addition to the desired signal. JD's ability to retrieveinterference signals allows the suppression of specific adjacent-channelinterferers. In addition to demodulating GMSK signals, JD also can beused to demodulate EDGE signals.

The second DARP method is blind interferer cancellation (BIC). With BIC,the receiver has no knowledge of the structure of any interferingsignals that may be received at the same time that the desired signal isreceived. Since the receiver is effectively “blind” to anyadjacent-channel interferers, the method attempts to suppress theinterfering component as a whole. The GMSK signal is demodulated fromthe wanted carrier by the BIC method. BIC is most effective when usedfor GMSK-modulated speech and data services and can be used inasynchronous networks.

A DARP capable remote station equalizer/detector 426 of the presentmethod and apparatus also performs CCI cancellation prior toequalization, detection, etc. Equalizer/detector 426 in FIG. 2 providesdemodulated data. CCI cancellation normally is available on a BS 110,111, 114. Also, remote stations 123-127 may or may not be DARP capable.The network may determine whether a remote station is DARP capable ornot at the resource assignment stage, a starting point of a call, orduring the power-on stage for a GSM remote station (e.g. mobilestation).

It is desirable to increase the number of active connections to remotestations that can be handled by a base station.

FIG. 5 of the accompanying drawings is a simplified representation of acellular communications system 100. The system comprises base stations110, 111 and 114 and remote stations 123, 124, 125, 126 and 127. Basestation controllers 141 to 144 act to route signals to and from thedifferent remote stations 123-127, under the control of mobile switchingcentres 151, 152. The mobile switching centres 151, 152 are connected toa public switched telephone network (PSTN) 162. Although remote stations123-127 are commonly handheld mobile devices, many fixed wirelessdevices and wireless devices capable of handling data also fall underthe general title of remote station 123-127.

Signals carrying, for example, voice data are transferred between eachof the remote stations 123-127 and other remote stations 123-127 bymeans of the base station controllers 141-144 under the control of themobile switching centres 151, 152. Alternatively, signals carrying, forexample, voice data are transferred between each of the remote stations123-127 and other communications equipment of other communicationsnetworks via the public switched telephone network 162. The publicswitched telephone network 162 allows calls to be routed between themobile cellular system 100 and other communication systems. Such othersystems include other mobile cellular communications systems 100 ofdifferent types and conforming to different standards.

Each of remote stations 123-127 can be serviced by any one of a numberof base stations 110, 111, 114. A remote station 124 receives both asignal transmitted by the serving base station 114 and signalstransmitted by nearby non-serving base stations 110, 111 and intended toserve other remote stations 125.

The strengths of the different signals from base stations 110, 111, 114are periodically measured by the remote station 124 and reported to BSC144, 114, etc. If the signal from a nearby base station 110, 111 becomesstronger than that of the serving base station 114, then the mobileswitching centre 152 acts to make the nearby base station 110 become theserving base station and acts to make the serving base station 114become a non-serving base station and handovers the signal to the nearbybase station 110. Handover refers to the method of transferring a datasession or an ongoing call from one channel connected to the corenetwork to another.

In cellular mobile communications systems, radio resources are dividedinto a number of channels. Each active connection (for example a voicecall) is allocated a particular channel having a particular channelfrequency for the downlink signal (transmitted by the base station 110,111, 114 to a remote station 123-127 and received by the remote station123-127) and a channel having a particular channel frequency for theuplink signal (transmitted by the remote station 123-127 to the basestation 110, 111, 114 and received by the base station 110, 111, 114).The frequencies for downlink and uplink signals are often different, toallow simultaneous transmission and reception and to reduce interferencebetween transmitted signals and the received signals at the remotestation or 123-127 at the base station 110, 111, 114.

FIG. 6 of the accompanying drawings shows an arrangement of cells in acellular communications system that uses frequency reuse, a method thatmay be used in a cellular system to provide access to many users of thesystem. This particular example has a reuse factor of 4:12, whichrepresents 4 sites and 12 frequencies. That means that the 12frequencies available for use by a base station are allocated to thebase stations of four sites labeled A-D, each site having one basestation 110, 111, 114. Each site is divided into three sectors (nowusually called cells). Stated another way, one frequency is allocated toeach of the three cells of each of 4 sites so that all of the 12 cellshave different frequencies. The frequency reuse pattern repeats itselfas shown in the figure. Base station 110 belongs to cell A, base station114 belongs to cell B, base station 111 belongs to cell C and so on.Base station 110 has a service area 220 that overlaps with adjacentservice areas 230 and 240 of adjacent base stations 111 and 114respectively. Remote stations 124, 125 are free to roam between theservice areas.

As discussed above, to reduce interference of signals between cells,each site is allocated a set of channel frequencies which is differentto the set of channel frequencies allocated to each of its neighboringsites. However, two sites that are non-adjacent may use the same set offrequencies. Base station 110 could use for example frequency allocationset A comprising frequencies f1, f2 and f3 for communicating with remotestations 125 in its service area 220. Similarly, base station 114 coulduse for example frequency allocation set B comprising frequencies f4, f5and f6, to communicate with remote stations 124 in its service area 240,and so on. The area defined by bold border 250 contains one four-siterepeat pattern. The repeat pattern repeats in a regular arrangement forthe geographical area serviced by the communications system 100. It maybe appreciated that although the present example repeats itself after 4sites, a repeat pattern may have a number of sites other than four and atotal number of frequencies other than 12.

TDMA is a multiple access technique directed to providing increasedcapacity. Using TDMA, each carrier frequency is segmented into timeintervals called frames. Each frame is further partitioned intoassignable user time slots. In GSM, the frame is partitioned into eighttime slots. Thus, eight consecutive time slots form one TDMA frame witha duration of 4.615 ms.

A physical channel occupies one time slot within each frame on aparticular frequency. The TDMA frames of a particular carrier frequencyare numbered, each user being assigned one or more time slots withineach frame. Furthermore, the frame structure repeats, so that a fixedTDMA assignment constitutes one or more slots that periodically appearduring each time frame. Thus, each base station can communicate with aplurality of remote stations 123-127 using different assigned time slotswithin a single channel frequency. As stated above, the time slotsrepeat periodically. For example, a first user may transmit on the1^(1st) slot of every frame of frequency f1, while a second user maytransmit on the 2^(nd) slot of every frame of frequency f2. During eachdownlink time slot, the remote station 123-127 is given access toreceive a signal transmitted by the base station 110, 111, 114 andduring each uplink time slot the base station 110, 111, 114 is givenaccess to receive a signal transmitted by the remote station 123-127.The channel for communication to a remote station 123-127 thus comprisesboth a frequency and a time slot, for a GSM system. Equally, the channelfor communication to a base station 110, 111, 114 comprises both afrequency and a time slot.

FIG. 7 shows an example arrangement of time slots for a time divisionmultiple access (TDMA) communications system. A base station 114transmits data signals in a sequence of numbered time slots 30, eachsignal being for only one of a set of remote stations 123-127 and eachsignal being received at the antenna of all remote stations 123-127within range of the transmitted signals. The base station 114 transmitsall the signals using slots on an allocated channel frequency. Forexample, a first remote station 124 might be allocated a first time slot3 and a second remote station 126 might be allocated a second time slot5.

The base station 114 transmits, in this example, a signal for the firstremote station 124 during time slot 3 of the sequence of time slots 30,and transmits a signal for the second remote station 126 during timeslot 5 of the sequence of time slots 30. The first and second remotestations 124, 126 are active during their respective time slots 3 and 5of time slot sequence 30, to receive the signals from the base station114. The remote stations 124, 126 transmit signals to the base station114 during corresponding time slots 3 and 5 of time slot sequence 31 onthe uplink. It can be seen that the time slots for the base station 114to transmit (and the remote stations 124, 126 to receive) 30 are offsetin time with respect to the time slots for the remote stations 124, 126to transmit (and the base station 114 to receive) 31.

This offsetting in time of transmit and receive time slots is known astime division duplexing (TDD), which among other things, allows transmitand receive operations to occur at different instances of time.

Voice and data signals are not the only signals to be transmittedbetween the base station 110, 111, 114 and the remote station 123-127. Acontrol channel is used to transmit data that controls various aspectsof the communication between the base station 110, 111, 114 and theremote station 123-127. Among other things, the base station 110, 111,114 uses the control channel to send to the remote station 123-127 asequence code, or training sequence code (TSC) which indicates which ofa set of sequences the base station 110, 111, 114 will use to transmitthe signal to the remote station 123-127. In GSM, a 26-bit trainingsequence is used for equalization. This is a known sequence which istransmitted in a signal in the middle of every time slot burst.

The sequences are used by the remote station 123-127 to compensate forchannel degradations which vary quickly with time; to reduceinterference from other sectors or cells; and to synchronize the remotestation's 123-127 receiver to the received signal. These functions areperformed by an equalizer which is part of the remote station's 123-127receiver. An equalizer 426 determines how the known transmitted trainingsequence signal is modified by multipath fading. Equalization may usethis information to extract the desired signal from the unwantedreflections by constructing an inverse filter to extract the rest of thedesired signal. Different sequences (and associated sequence codes) aretransmitted by different base stations 110, 111, 114 in order to reduceinterference between sequences transmitted by base stations 110, 111,114 that are close to each other.

As stated above, with DARP the remote station 123-127 of the presentmethod and apparatus is able to use the sequence to distinguish thesignal transmitted to it by the base station 110, 111, 114 serving theremote station 123-127 from other unwanted signals transmitted bynon-serving base stations 110, 111, 114 of other cells. This holds trueso long as the received amplitudes or power levels of the unwantedsignals are below a threshold relative to the amplitude of the wantedsignal. The unwanted signals can cause interference to the wanted signalif they have amplitudes above this threshold. In addition, the thresholdcan vary according to the capability of the remote station's 123-127receiver. The interfering signal and the desired (or wanted) signal canarrive at the remote station's 123-127 receiver contemporaneously if,for example, the signals from the serving and non-serving base stations110, 111, 114 share the same time slot for transmitting.

Referring again to FIG. 5, at remote station 124, transmissions frombase station 110 for remote station 125 can interfere with transmissionsfrom base station 114 for remote station 124 (the path of theinterfering signal shown by dashed arrow 170). Similarly, at remotestation 125 transmissions from base station 114 for remote station 124can interfere with transmissions from base station 110 for remotestation 125 (the path of the interfering signal shown by dotted arrow182).

TABLE 1

Table 1 shows example values of parameters for signals transmitted bythe two base stations 110 and 114 illustrated in FIG. 6. The informationin rows 3 and 4 of Table 1 show that for remote station 124 both awanted signal from a first base station 114 and an unwanted interferersignal from a second base station 110 and intended for remote station125 are received and the two received signals have the same channel andsimilar power levels (−82 dBm and −81 dBm respectively). Similarly, theinformation in rows 6 and 7 show that for remote station 125 both awanted signal from the second base station 110 and an unwantedinterferer signal from the first base station 114 and intended forremote station 124 are received and the two received signals have thesame channel and similar power levels (−80 dBm and −79 dBmrespectively).

Each remote station 124, 125 thus receives both a wanted signal and anunwanted interferer signal that have similar power levels from differentbase stations 114, 110, on the same channel (i.e. contemporaneously).Because the two signals arrive on the same channel and similar powerlevels, they interfere with each other. This may cause errors indemodulation and decoding of the wanted signal. This interference isco-channel interference discussed above.

The co-channel interference may be mitigated to a greater extent thanpreviously possible, by the use of DARP enabled remote stations 123-127,base stations 110, 111, 114 and base station controllers 151, 152. Whilebase stations 110, 111, 114 may be capable of simultaneously receivingand demodulating two co-channel signals having similar power levels,DARP allows remote stations 123-127 to have, by means of DARP, similarcapability. This DARP capability may be implemented by means of SAIC orby means of a method known as dual antenna interference cancellation(DAIC).

The receiver of a DARP-capable remote station 123-127 may demodulate awanted signal while rejecting an unwanted co-channel signal even whenthe amplitude of the received unwanted co-channel signal is similar orhigher than the amplitude of the wanted signal. The DARP feature worksbetter when the amplitudes of the received co-channel signals aresimilar. This situation would typically occur in existing systems suchas GSM not yet employing the present method and apparatus, when each oftwo remote stations 123-127, each communicating with a different basestation 110, 111, 114, is near a cell boundary, where the path lossesfrom each base station 110, 111, 114 to each remote station 123-127 aresimilar.

A remote station 123-127 that is not DARP-capable, by contrast, may onlydemodulate the wanted signal if the unwanted co-channel interferersignal has an amplitude, or power level, lower than the amplitude of thewanted signal. In one example, it may be lower by at least 8 dB. TheDARP-capable remote station 123-127 can therefore tolerate a muchhigher-amplitude co-channel signal relative to the wanted signal, thancan the remote station 123-127 not having DARP capability.

The co-channel interference (CCI) ratio is the ratio between the powerlevels, or amplitudes, of the wanted and unwanted signals expressed indB. In one example, the co-channel interference ratio could be, forexample, −6 dB (whereby the power level of the wanted signal is 6 dBlower than the power level of the co-channel interferer (or unwanted)signal). In another example, the ratio may be +6 dB (whereby the powerlevel of the wanted signal is 6 dB higher than the power level of theco-channel interferer (or unwanted) signal). For those remote stations123-127 of the present method and apparatus with good DARP performance,the amplitude of the interferer signal can be as much as 10 dB higherthan the amplitude of the wanted signal, and the remote stations 123-127may still process the wanted signal. If the amplitude of the interferersignal is 10 dB higher than the amplitude of the wanted signal, theco-channel interference ratio is −10 dB.

DARP capability, as described above, improves a remote station's 123-127reception of signals in the presence of ACI or CCI. A new user, withDARP capability, will better reject the interference coming from anexisting user. The existing user, also with DARP capability, would dothe same and not be impacted by the new user. In one example, DARP workswell with CCI in the range of 0 dB (same level of co-channelinterference for the signals) to −6 dB (co-channel is 6 dB stronger thanthe desired or wanted signal). Thus, two users using the same ARFCN andsame timeslot, but assigned different TSCs, will get good service.

The DARP feature allows two remote stations 124 and 125, if they bothhave the DARP feature enabled, to each receive wanted signals from twobase stations 110 and 114, the wanted signals having similar powerlevels, and each remote station 124, 125 to demodulate its wantedsignal. Thus, the DARP enabled remote stations 124, 125 are both able touse the same channel simultaneously for data or voice.

The feature described above of using a single channel to support twosimultaneous calls from two base stations 110, 111, 114 to two remotestations 123-127 is somewhat limited in its application in the priorart. To use the feature, the two remote stations 124, 125 are withinrange of the two base stations 114, 110 and are each receiving the twosignals at similar power levels. For this condition, typically the tworemote stations 124, 125 would be near the cell boundary, as mentionedabove.

The present method and apparatus allows the supporting of two or moresimultaneous calls on the same channel (consisting of a time slot on acarrier frequency), each call comprising communication between a singlebase station 110, 111, 114 and one of a plurality of remote stations123-127 by means of a signal transmitted by the base station 110, 111,114 and a signal transmitted by the remote station 123-127. The presentmethod and apparatus provides a new and inventive application for DARP.As stated above, with DARP, two signals on the same time slot on thesame carrier frequency may be distinguished by using different trainingsequences at higher levels of interference than before DARP. Since thesignal from the BS 110, 111, 114 not being used acts as interference,DARP filters/suppresses out the unwanted signal (signal from the BS 110,111, 114 not being used) by use of the training sequences.

The present method and apparatus allows the use of two or more trainingsequences in the same cell. In the prior art, one of the trainingsequences, the one not assigned to the base station 110, 111, 114, willonly act as interference as it also does in Multi-User on One Slot(MUROS) for at least one mobile station's 123-127 receiver. However, akey difference is that the unwanted signal for that mobile station123-127 is wanted by another mobile station 123-127 in the same cell. Inlegacy systems, the unwanted signal is for a mobile station 123-127 inanother cell. According to the present method and apparatus, bothtraining sequence signals may be used in the same time slot on the samecarrier frequency in the same cell by the same base station 110, 111,114. Since two training sequences can be used in a cell, twice as manycommunication channels may be used in the cell. By taking a trainingsequence which would normally be interference from another(non-neighboring) cell or sector and allowing a base station 110, 111,114 to use it in addition to its already-used training sequence for thesame time slot, the number of communication channels is doubled. Ifthree training sequences are used in the same time slot in this way, thenumber of communication channels is tripled.

DARP, when used along with the present method and apparatus, thereforeenables a GSM network to use a co-channel already in use (i.e., theARFCN that is already in use) to serve additional users. In one example,each ARFCN can be used for two users for full-rate (FR) speech and 4 forhalf-rate (HR) speech. It is also possible to serve the third or evenfourth user if the remote stations 123-127 have excellent DARPperformance.

In order to serve additional users using the same AFRCN on the sametimeslot, the network transmits the additional users' RF signal on thesame carrier, using a different phase shift, and assigns the sametraffic channel (the same ARFCN and timeslot that is in use) to theadditional user using a different TSC. The bursts are modulated with thetraining sequence corresponding to the TSC accordingly. A DARP capableremote station 123-127 may detect the wanted or desired signal. It ispossible to add the third and fourth users in the same way as the firstand second users were.

FIG. 8A of the accompanying drawings shows an apparatus for operating ina multiple access communication system to produce first and secondsignals sharing a single channel. A first data source 401 and a seconddata source 402 (for a first and a second remote station 123-127)produce first data 424 and second data 425 for transmission. A sequencegenerator 403 generates a first sequence 404 and a second sequence 405.A first combiner 406 combines the first sequence 404 with the first 424data to produce first combined data 408. A second combiner 407 combinesthe second sequence 405 with the second data 425 to produce secondcombined data 409.

The first and second combined data 408, 409 are input to a transmittermodulator 410 for modulating both the first and the second combined data408, 409 using a first carrier frequency 411 and a first time slot 412.In this example, the carrier frequency may generated by an oscillator421. The transmitter modulator outputs a first modulated signal 413 anda second modulated signal 414 to a RF front end 415. The RF front endprocesses the first and second modulated signals 413, 414 byupconverting them from baseband to an RF (radio frequency) frequency.The upconverted signals are sent to antennas 416 and 417 where they arerespectively transmitted.

The first and second modulated signals may be combined in a combinerprior to being transmitted. The combiner 422 may be a part of either thetransmitter modulator 410 or the RF front end 415 or a separate device.A single antenna 416 provides means for transmitting the combined firstand second signals by radiation. This is illustrated in FIG. 8B.

FIG. 9 of the accompanying drawings is a flowchart of a method of usingthe apparatus shown in FIG. 8A or 8B of the accompanying drawings. Themethod includes allocating a particular channel frequency and aparticular time slot for a base station 110, 111, 114 to use to transmitto a plurality of remote stations 123-127 whereby a different trainingsequence is assigned for each remote station 123-127. Thus in oneexample, this method may be executed in the base station controller 151,152. In another example, this method may be executed in a base station110, 111, 114.

Following the start of the method 501, a decision is made in step 502 asto whether to set up a new connection between the base station 110, 111,114 and a remote station 123-127. If the answer is NO, then the methodmoves back to the start block 501 and the steps above are repeated. Whenthe answer is YES, a new connection is set up. Then in block 503 adecision is made as to whether there is an unused channel (i.e. anunused time slot for any channel frequency). If there is an unused timeslot on a used or unused channel frequency, then a new time slot isallocated in block 504. The method then moves back to the start block501 and the steps above are repeated.

When eventually there is no longer an unused time slot (because all timeslots are used for connections), the answer to the question of block 503is NO, and the method moves to block 505. In block 505, a used time slotis selected for the new connection to share with an existing connection,according to a set of first criteria. There can be a variety ofcriteria. For example one criterion might be that a time slot may beselected if it has low traffic. Another criterion may be that the timeslot is already used by no more than one remote station 123-127. It canbe appreciated that there will be other possible criteria based on thenetwork planning methods employed, and the criteria is not limited tothose two examples.

A used time slot on a channel frequency having been selected for the newconnection to share along with an existing connection, a TSC for the newconnection is then selected in block 506 according to a set of secondcriteria. These second criteria may include some of the criteria usedfor the selection of the time slot in block 505, or other criteria. Onecriterion is that the TSC has not yet been used by the cell or sectorfor the channel comprising the used time slot. Another criterion mightbe that the TSC is not used on that channel by a nearby cell or sector.The method then moves back to the start block 501 and the steps aboveare repeated.

FIG. 10A of the accompanying drawings shows an example embodimentwherein the method described by FIG. 9 would reside in the base stationcontroller 600. Within base station controller 600 reside controllerprocessor 660 and memory subsystem 650. The steps of the method may bestored in software 680 in memory 685 in memory subsystem 650, or withinsoftware 680 in memory 685 residing in controller processor 660, orwithin software 680 memory 685 in the base station controller 600, orwithin some other digital signal processor (DSP) or in other forms ofhardware. The base station controller 600 is connected to the mobileswitching centre 610 and also to base stations 620, 630 and 640, asshown by FIG. 10A.

Shown within memory subsystem 650 are parts of three tables of data 651,652, 653. Each table of data stores values of a parameter for a set ofremote stations 123, 124 indicated by the column labeled MS. Table 651stores values of training sequence code. Table 652 stores values fortime slot number TS. Table 653 stores values of channel frequency CHF.It can be appreciated that the tables of data could alternatively bearranged as a multi-dimensional single table or several tables ofdifferent dimensions to those shown in FIG. 10A.

Controller processor 660 communicates via data bus 670 with memorysubsystem 650 in order to send and receive values for parameters to/frommemory subsystem 650. Within controller processor 660 are containedfunctions that include a function 661 to generate an access grantcommand, a function 662 to send an access grant command to a basestation 620, 630, 640, a function 663 to generate a traffic assignmentmessage, and a function 664 to send a traffic assignment message to abase station 620, 630 or 640. These functions may be executed usingsoftware 680 stored in memory 685.

Within controller processor 660, or elsewhere in the base stationcontroller 600, there may also be a power control function 665 tocontrol the power level of a signal transmitted by a base station 620,630 or 640.

It can be appreciated that the functions shown as being within basestation controller 600, namely memory subsystem 650 and controllerprocessor 660 could also reside in the mobile switching centre 610.Equally some or all of the functions described as being part of basestation controller 600 could equally well reside in one or more of basestations 620, 630 or 640.

FIG. 10B is a flowchart illustrating the steps executed by the basestation controller 600 of FIG. 10A. When allocating a channel to aremote station 123, 124 (e.g. remote station MS 123), for example whenthe remote station 123 requests service, the base station 620, 630, 640wishing to service the remote station 123, 124 sends a request messageto the base station controller 600 for a channel assignment. Controllerprocessor 660, upon receiving the request message at step 602 via databus 670, determines if a new connection is required. If the answer isNO, then the method moves back to the start block 601 and the stepsabove are repeated. When the answer is YES a new connection set up isinitiated. Then in block 603 a decision is made as to whether there isan unused channel (i.e. an unused time slot for any channel frequency).If there is an unused time slot on a used or unused channel frequency,then a new time slot is allocated in block 604. The method then movesback to the start block 601 and the steps above are repeated.

On the other hand, if the controller processor 660 determines there isnot an unused time slot on any channel frequency, it selects a used timeslot. See step 605 of FIG. 10B. The selection could be based onaccessing memory subsystem 650 or other memory 685 to obtain informationon criteria such as the current usage of time slots, and whether both oronly one of remote stations 123, 124 are DARP enabled. Controllerprocessor 660 selects a used time slot, and selects a training sequencecode for the time slot. See step 606 of FIG. 10B Since the time slot isalready used, this will be the second training sequence selected forthat time slot.

In order to apply criteria for selecting a time slot, the controllerprocessor 660 accesses memory 650 via data bus 670, or accesses othermemory 685, to obtain information, for example information about thecurrent allocation of time slots or training sequences or both, andwhether remote stations 123, 124 have DARP capability. Controllerprocessor 660 then generates a command (661 or 663) and sends thecommand (662 or 664) to the base station 620 to assign a channelfrequency, time slot and training sequence to the remote station 123.The method then moves back to the start block 601 and the steps aboveare repeated.

FIG. 11 of the accompanying drawings shows the flow of signals in a basestation 620, 920. Base station controller interface 921 communicates,via communications link 950, with a base station controller 600.Communications link 950 might be a data cable or a RF link for example.Controller processor 960 communicates with and controls, via data bus970, receiver components 922, 923 and 924, and transmitter components927, 928, and 929. Controller processor 960 communicates via data bus980 with BSC interface 921. The data bus 970 could comprise just one busor several buses and could be partly or wholly bi-directional. Databuses 970 and 980 could be the same bus.

In one example, a message requesting grant of a channel is received froma remote station 123, 124 in a coded, modulated, radiated signal at basestation antenna 925 and is input to duplexer switch 926. The signalpasses from the receive port of duplexer switch 926 to the receiverfront end 924 which conditions the signal (for example by means ofdown-converting, filtering, and amplifying). The receiver demodulator923 demodulates the conditioned signal and outputs the demodulatedsignal to channel decoder and de-interleaver 922 which decodes andde-interleaves the demodulated signal and outputs the resulting data tocontroller processor 960. Controller processor 960 derives from theresulting data the message requesting grant of a channel. Controllerprocessor 960 sends the message via base station controller interface921 to a base station controller 600. The base station controller 600then acts to grant, or not grant, a channel to the remote station 23,24, either autonomously or together with mobile switching centre 610.

Base station controller 600 generates and sends access grant commands,and other digital communication signals or traffic for remote stations123, 124, for example assignment messages, to BSC interface 921 viacommunications link 950. The signals are then sent via data bus 980 tocontroller processor 960. Controller processor 960 outputs signals forremote stations 123, 124 to coder and interleaver 929 and the coded andinterleaved signals then pass to transmitter modulator 928. It can beseen from FIG. 11 that there are several signals input to transmittermodulator 928, each signal for a remote station 123, 124. These severalsignals can be combined within transmitter modulator 928 to provide acombined modulated signal having I and Q components as shown in FIG. 11.

However the combining of the several signals could alternatively beperformed post-modulation within transmitter front end module 927 and orin other stages within the transmit chain. The modulated combined signalis output from transmitter front end 927 and input to the transmit portof duplexer switch 926. The signal is then output via the common orantenna port of duplexer switch 926 to the antenna 925 for transmission.

In another example, a second message from a second remote station 123,124 requesting grant of a channel is received in a second receivedsignal at the base station antenna 925. The second received signal isprocessed as described above and the request for grant of a channel issent in the processed second received signal to the base stationcontroller 600.

The base station controller 600 generates and sends to the base station620, 920 a second access grant message as described above, and the basestation 620, 920 transmits a signal comprising the second access grantmessage, as described above, for the remote station 123, 124.

Phase Shift

The absolute phase of the modulation for the two signals transmitted bythe base station 110, 111, 114 may not be identical. In order to serveadditional users using the same channel (co-TCH), in addition toproviding more than one TSC, the network may phase shift the symbols ofthe RF signal of the new co-channel (co-TCH) remote station 123-127 withrespect to the existing co-TCH remote station(s) 123-127. If possiblethe network may control them with evenly distributed spaced phase shift,thus improving receiver performance.

For example, the phase shift of the carrier frequency (having aparticular ARFCN) for two users would be 90 degrees apart, three users60 degrees apart. The phase shift of the carrier (ARFCN) for four userswould be 45 degree apart. As stated above, the users will use differentTSCs. Each additional MS 123-127 of the present method and apparatus isassigned a different TSC and uses its own TSC and the DARP feature toget its own traffic data.

Thus, for improved DARP performance, the two signals intended for thetwo different mobile stations (remote stations) 123, 124 may ideally bephase shifted by π/2 for their channel impulse response, but less thanthis will also provide adequate performance.

When the first and second remote stations 123, 124 are assigned the samechannel (i.e. same time slot on the same channel frequency), signals maypreferably be transmitted to the two remote stations 123, 124 (usingdifferent training sequences as described previously) such that themodulator 928 modulates the two signals at 90 degrees phase shift toeach other, thus further reducing interference between the signals dueto phase diversity. So, for example, the I and Q samples emerging fromthe modulator 928 could each represent one of the two signals, thesignals being separated by 90 degrees phase. The modulator 928 thusintroduces a phase difference between the signals for the two remotestations 123, 124.

In the case of several remote stations 123, 124 sharing the samechannel, multiple sets of I and Q samples can be generated withdifferent offsets. For example, if there is a third signal for a thirdremote station 123, 124 on the same channel, the modulator 928introduces phase shifts of preferably 60 degrees and 120 degrees for thesecond and third signals relative to the phase of the first signal, andthe resulting I and Q samples represent all three signals. For example,the I and Q samples could represent the vector sum of the three signals.

In this way, the transmitter modulator 928 provides means at the basestation 620, 920 for introducing a phase difference betweencontemporaneous signals using the same time slot on the same frequencyand intended for different remote stations 123, 124. Such means can beprovided in other ways. For example, separate signals can be generatedin the modulator 928 and resulting analogue signals can be combined inthe transmitter front end 927 by passing one of them through a phaseshift element and then simply summing the phase shifted and non-phaseshifted signals.

Power Control Aspects

Table 2 below shows example values of channel frequency, time slot,training sequence and received signal power level for signalstransmitted by the two base stations 110 and 114 as shown in FIG. 5 andreceived by remote stations 123 to 127.

TABLE 2

The rows 3 and 4 of Table 2, outlined by a bold rectangle, show bothremote station 123 and remote station 124 using channel frequency havingindex 32 and using time slot 3 for receiving a signal from base station114 but allocated different training sequences TSC2 and TSC3respectively. Similarly, rows 9 and 10 also show the same channelfrequency and time slot being used for two remote stations 125, 127 toreceive signals from the same base station 110. It can be seen that ineach case the remote station 125, 127 received power levels of thewanted signals are substantially different for the two remote stations125, 127.

The highlighted rows 3 and 4 of Table 3 show that base station 114transmits a signal for remote station 123 and also transmits a signalfor remote station 124. The received power level at remote station 123is −67 dBm whereas the received power level at remote station 124 is−102 dBm. Rows 9 and 10 of Table 3 show that base station 110 transmitsa signal for remote station 125 and also transmits a signal for remotestation 127. The received power level at remote station 125 is −101 dBmwhereas the received power level at remote station 127 is −57 dBm. Thelarge difference in power level, in each case, could be due to differentdistances of the remote stations 125, 127 from the base station 110.Alternatively the difference in power levels could be due to differentpath losses or different amounts of multi-path cancellation of thesignals, between the base station transmitting the signals and theremote station receiving the signals, for one remote station as comparedto the other remote station.

Although this difference in received power level for one remote stationcompared to the other remote station is not intentional and not idealfor cell planning, it does not compromise the operation of the presentmethod and apparatus.

A remote station 123-127 having DARP capability may successfullydemodulate either one of two co-channel, contemporaneously receivedsignals, so long as the amplitudes or power levels of the two signalsare similar at the remote station's 123-127 antenna. This is achievableif the signals are both transmitted by the same base station 110, 111,114 and (could have more than one antenna, e.g., one per signal) thepower levels of the two transmitted signals are substantially the samebecause then each remote station 123-127 receives the two signals atsubstantially the same power level (say within 6 dB of each other). Thetransmitted powers are similar if either the base station 110, 111, 114is arranged to transmit the two signals at similar power levels, or thebase station 110, 111, 114 transmits both signals at a fixed powerlevel. This situation can be illustrated by further reference to Table 2and by reference Table 3.

While Table 2 shows remote stations 123, 124 receiving from base station114 signals having substantially different power levels, on closerinspection it can be seen that, as shown by rows 3 and 5 of Table 2,remote station 123 receives two signals from base station 114 at thesame power level (−67 dBm), one signal being a wanted signal intendedfor remote station 123 and the other signal being an unwanted signalwhich is intended for remote station 124. The criteria for a remotestation 123-127 to receive signals having similar power levels is thusshown as being met in this example. If mobile station 123 has a DARPreceiver, it can, in this example, therefore demodulate the wantedsignal and reject the unwanted signal.

Similarly, it can be seen by inspecting rows 4 and 6 of Table 2 (above)that remote station 124 receives two signals sharing the same channeland having the same power level (−102 dBm). Both signals are from basestation 114. One of the two signals is the wanted signal, for remotestation 124 and the other signal is the unwanted signal which isintended for use by remote station 123.

To further illustrate the above concepts, Table 3 is an altered versionof Table 2 wherein the rows of Table 2 are simply re-ordered. It can beseen that remote stations 123 and 124 each receive from one base station114 two signals, a wanted and an unwanted signal, having the samechannel and similar power levels. Also, remote station 125 receives fromtwo different base stations 110, 114 two signals, a wanted and anunwanted signal, having the same channel and similar power levels.

TABLE 3

The apparatus and method described above have been simulated and themethod has been found to work well in a GSM system. The apparatusdescribed above and shown in FIGS. 8A, 8B, 10A, 11 and 12 could be partof a base station 110, 111, 114 of a GSM system for example.

According to another aspect of the present method and apparatus it ispossible for a base station 110, 111, 114 to maintain a call with tworemote stations 123-127 using the same channel, such that a first remotestation 123-127 has a DARP-enabled receiver and a second remote station123-127 does not have a DARP-enabled receiver. The amplitudes of signalsreceived by the two remote stations 124-127 are arranged to be differentby an amount which is within a range of values, in one example it may bebetween 8 dB and 10 dB, and also arranged such that the amplitude of thesignal intended for the DARP-enabled remote station is lower than theamplitude of the signal intended for the non-DARP-enabled remote station124-127.

A MUROS or non-MUROS mobile may treat its unwanted signal asinterference. However, for MUROS, both signals may be treated as wantedsignals in a cell. An advantage with MUROS enabled networks (thenetworks including e.g., a BS 110, 111, 114 and BSC 141, 144) is thatthe BS 110, 111, 114 may use two or more training sequences per timeslotinstead of only one so that both signals may be treated as desiredsignals in the same cell. The BS 110,111, 114 transmits the signals atsuitable amplitudes so that each remote station 123-127 of the presentmethod and apparatus receives its own signal at a high enough amplitudeand the two signals are maintained with an amplitude ratio such that thetwo signals corresponding to the two training sequences may be both bedetected.

This feature may be implemented using software stored in memory in theBS 110, 111, 114 or BSC 600. For example, MSs 123-127 are selected forpairing based on their path losses and based on existing traffic channelavailability. However, MUROS can still work if the path losses are verydifferent for one remote station 123-127 than for the other remotestation 123-127. This may occur when one remote station 123-127 is muchfurther away from the BS 110, 111, 114.

Regarding power control there are different possible combinations ofpairings. Both remote stations 123-127 can be DARP capable oralternatively only one can be DARP capable. In both cases, the receivedamplitudes or power levels at the mobile stations 123-127 may be within10 dB of each other. However if only one remote station 123-127 is DARPcapable, a further constraint is that the non-DARP mobile 123-127receives its wanted (or desired) first signal at a level higher than thelevel at which it receives the second signal (in one example, at least 8dB higher than the second signal). The DARP capable remote station123-127 receives its second signal at a level which is lower than thelevel of the first signal by an amount which is less than a thresholdamount (in one example, the second signal is no lower than 10 dB belowthe first signal).

Hence in one example, the amplitude ratio can be 0 dB to ±10 dB for twoDARP capable remote stations 123-127 or, in the case of a non-DARP/DARPpairing of remote stations 123-127, the signal for the non-DARP remotestation 123-127 is received 8 dB to 10 dB higher than the signal for theDARP remote station 123-127. Also, it is preferable for the BS 110, 111,114 to transmit the two signals so that each remote station 123-127receives its wanted signal above its sensitivity limit. (In one example,it is at least 6 dB above its sensitivity limit). So if one remotestation 123-127 has more path loss, the BS 110, 111, 114 transmits thatremote station's 123-127 signal at an amplitude high enough to ensurethat the transmitted signal is received by the remote station 123-127 ata level above the sensitivity limit. This sets the absolute transmittedamplitude for that signal. The difference in level required between thatsignal and the other signal then determines the absolute amplitude ofthe other signal.

FIG. 12 of the accompanying drawings shows example arrangements for datastorage within a memory subsystem 650 which might reside within a basestation controller (BSC) 600 of the present method and apparatus ofcellular communication system 100. Table 1001 of FIG. 12 is a table ofvalues of channel frequencies assigned to remote stations 123-127, theremote stations 123-127 being numbered. Table 1002 is a table of valuesof time slots wherein remote station numbers 123-127 are shown againsttime slot number. It can be seen that time slot number 3 is assigned toremote stations 123, 124 and 229. Similarly table 1003 shows a table ofdata allocating training sequences (TSCs) to remote stations 123-127.

Table 1005 of FIG. 12 shows an enlarged table of data which ismulti-dimensional to include all of the parameters shown in tables 1001,1002, and 1003 just described. It will be appreciated that the portionof table 1005 shown in FIG. 12 is only a small part of the completetable that would be used. Table 1005 shows in addition the allocation offrequency allocation sets, each frequency allocation set correspondingto a set of frequencies used in a particular sector of a cell or in acell. In Table 1005, frequency allocation set f1 is assigned to allremote stations 123-127 shown in the table 1005 of FIG. 12. It will beappreciated that other portions of Table 1005, which are not shown, willshow frequency allocation sets f2, f3 etc. assigned to other remotestations 123-127. The fourth row of data shows no values but repeateddots indicating that there are many possible values not shown betweenrows 3 and 5 of the data in table 1001.

FIG. 13 of the accompanying drawings shows an example receiverarchitecture for a remote station 123-127 of the present method andapparatus having the DARP feature. In one example, the receiver isadapted to use either the single antenna interference cancellation(SAIC) equalizer 1105, or the maximum likelihood sequence estimator(MLSE) equalizer 1106. Other equalizers implementing other protocols mayalso be used. The SAIC equalizer is preferred for use when two signalshaving similar amplitudes are received. The MLSE equalizer is typicallyused when the amplitudes of the received signals are not similar, forexample when the wanted signal has an amplitude much greater than thatof an unwanted co-channel signal.

FIG. 14 of the accompanying drawings shows part of a GSM system adaptedto assign the same channel to two remote stations 123-127. The systemcomprises a base station transceiver subsystem (BTS), or base station110, and two remote stations, mobile stations 125 and 127. The networkcan assign, via the base station transceiver subsystem 110, the samechannel frequency and the same time slot to the two remote stations 125and 127. The network allocates different training sequences to the tworemote stations 125 and 127. Remote stations 125 and 127 are both mobilestations and are both assigned a channel frequency having ARFCN equal to160 and a time slot with time slot index number, TS, equal to 3. Remotestation 125 is assigned training sequence having a TSC of 5 whereasremote station 127 is assigned training sequence having a TSC of 0. Eachremote station 125, 127 will receive its own signal (shown by solidlines in the figure) together with the signal intended for the otherremote station 125, 127 (shown by dotted lines in the figure). Eachremote station 125, 127 is able to demodulate its own signal whilstrejecting the unwanted signal.

As described above, according to the present method and apparatus asingle base station 110, 111, 114 can transmit a first and secondsignal, the signals for first and second remote stations 123-127respectively, each signal transmitted on the same channel, and eachsignal having a different training sequence. The first remote station123-127 having DARP capability is able to use the training sequences todistinguish the first signal from the second signal and to demodulateand use the first signal, when the amplitudes of the first and secondsignals are substantially within, say, 10 dB of each other.

In summary, FIG. 14 shows that the network assigns the same physicalresources to two mobile stations 125, 127, but allocates differenttraining sequences to them. Each MS will receive its own signal (shownas a solid line in FIG. 14) and that intended for the MS of the otherco-TCH user (shown as a dotted line in FIG. 14). On the downlink, eachmobile station will consider the signal intended for the other mobilestation as a CCI and reject the interference. Thus, two differenttraining sequences may be used to allow the suppression of interferencefrom a signal for another MUROS user.

Joint Detection on the Uplink

The present method and apparatus uses GMSK and the DARP capability ofthe handset to avoid the need for the network to support a newmodulation method. A network may use existing methods on the uplink toseparate each user, e.g., joint detection. It uses co-channel assignmentwhere the same physical resources are assigned to two different remotestations 123-127, but each mobile is assigned a different trainingsequence. On the uplink each remote station 123-127 of the presentmethod and apparatus may use a different training sequence. The networkmay use a joint detection method to separate two users on the uplink.

Speech Codec and Distance to New User

To reduce the interference to other cells, the BS 110, 111, 114 controlsits downlink power relative to the remote or mobile station's distancefrom it. When the MS 123-127 is close to the BS 110, 111, 114, the RFpower level transmitted by the BS 110, 111, 114 to the remote station123-127 on the downlink may be lower than to remote stations 123-127that are further away from the BS 110, 111, 114. The power levels forthe co-channel users are large enough for the caller who is further awaywhen they share the same ARFCN and timeslot. They can both have the samelevel of the power, but this can be improved if the network considersthe distance of co-channel users from the base station 110, 111, 114.

In one example, power may be controlled by identifying the distance andestimate the downlink power needed for the new user 123-127. This can bedone through the timing advance (TA) parameter of each user 123-127.Each user's 123-127 RACH provides this info to the BS 110, 111, 114.

Similar Distances for Users

Another novel feature is to pick a new user with a similar distance as acurrent/existing user. The network may identify the traffic channel(TCH=ARFCN and TS) of an existing user who is in the same cell and atsimilar distance and needs roughly the same power level identifiedabove. Also, another novel feature is that the network may then assignthis TCH to the new user with a different TSC from the existing user ofthe TCH.

Selection of Speech Codec

Another consideration is that the CCI rejection of a DARP capable mobilewill vary depending on which speech codec is used. Thus, the network(NW) may use this criteria and assign different downlink power levelsaccording to the distance to the remote station 123-127 and the codecsused.

Thus, it may be better if the network finds co-channel users who are ofsimilar distance to the BS 110, 111, 114. This is due to the performancelimitation of CCI rejection. If one signal is too strong compared to theother, the weaker signal may not be detected due to the interference.Therefore, the network may consider the distance from the BS 110, 111,114 to new users when assigning co-channels and co-timeslots. Thefollowing are procedures which the network may execute to minimize theinterference to other cells:

Frequency Hopping to Achieve User Diversity and Take Full Advantage ofDTx

Voice calls can be transmitted with a DTx (discontinuous transmission)mode. This is the mode that the allocated TCH burst can be quiet for theduration of no speech (while one is listening). The benefit of that whenevery TCH in the cell uses DTx is to reduce the overall power level ofthe serving cell on both UL and DL, hence the interference to others canbe reduced. This has significant effect, as normally people do have 40%of time listening. The DTx feature can be used in MUROS mode as well toachieve the know benefit as stated.

There is an extra benefit for MUROS to be achieved when frequencyhopping is used to establish user diversity. When two MUROS users pairtogether, there could be some period of time both MUROS paired users arein DTx. Although this is a benefit to other cells as stated above,neither of the MUROS paired users get the benefit from each other. Forthis reason, when both are in DTx, the allocated resources are wasted.To take the advantage of this potentially helpful DTx period, one canlet frequency hopping to take place so that a group of users are pairingwith each other dynamically on every frame basis. This method introducesuser diversity into the MUROS operation, and reduces the probabilitythat both paired MUROS users are in DTx. It also increases theprobability of having one GMSK on the TCH. Benefits include increasingthe performance of speech calls and maximizing the overall capacity ofthe network (NW).

An example of such case can be illustrated: Suppose the NW identified 8MUROS callers using full rate speech codecs, A, B, C, D, T, U, V, W, whouse similar RF power. Callers A, B, C, D can be non-frequency hopping.In addition, callers A, B, C, D are on the same timeslot, say TS3, butuse four different frequencies, ARFCN f1, f2, f3 and f4. Callers T, U,V, W are frequency hopping. In addition, callers T, U, V, W are on thesame timeslot TS3 and use frequencies f1, f2, f3 and f4 (MobileAllocation (MA) list). Suppose they are given Hopping Sequence Number(HSN)=0, and Mobile Allocation Index Offset (MAIO) 0, 1, 2 and 3respectively. This will let A, B, C, D pair with T, U, V, W in a cyclicform as shown in the table below.

Frame No. 0 1 2 3 4 5 6 7 8 9 10 11 f1 A/T A/W A/V A/U A/T A/W A/V A/UA/T A/W A/V A/U f2 B/U B/T B/W B/V B/U B/T B/W B/V B/U B/T B/W B/V f3C/V C/U C/T C/W C/V C/U C/T C/W C/V C/U C/T C/W f4 D/W D/V D/U D/T D/WD/V D/U D/T D/W D/V D/U D/T

The above is only an example. This form is selected to show how itworks. However it should not be limited to this particular arrangement.It works even better if more randomness of pairing is introduced. Thiscan be achieved by put all of 8 users on frequency hopping on the fourMA list, and give them different HSNs (in the above example 0 to 3) andMAIOs, provided two users are on each ARFCN.

Data Transfer

The first method pairs the traffic channel (TCH) being used. In oneexample, this feature is implemented on the network side, with minor orno changes made on the remote station side 123-127. The networkallocates a TCH to a second remote station 123-127 that is already inuse by a first remote station 123-127 with a different TSC. For example,when all the TCHs have been used, any additional service(s) requiredwill be paired with the existing TCH(s) that is (are) using similarpower.

For example, if the additional service is a 4D1U data call, then thenetwork finds four existing voice call users that use four consecutivetimeslots with similar power requirement to the additional new remotestation 123-127. If there is no such match, the network can reconfigurethe timeslot and ARFCN to make a match. Then the network assigns thefour timeslots to the new data call which needs 4D TCH. The new datacall also uses a different TSC. In addition, the uplink power for theadditional one may brought to be close or to equal the uplink power ofthe remote station 123-127 already using the timeslot.

Assigning a Remote Station 123-127 More than One TSC

If considering data services which use more than one timeslot, all (whenit is even) or all but one (when it is odd) of the timeslots may bepaired. Thus, improved capacity may be achieved by giving the remotestation 123-127 more than one TSC. By using multiple TSCs, the remotestation 123-127 may, in one example, combine its paired timeslots intoone timeslot so that the actual RF resource allocation may be cut byhalf.

For example, for 4DL data transfer, suppose that the remote station123-127 currently has bursts B1, B2, B3 and B4 in TS1, TS2, TS3 and TS4in each frame. Using the present method, B1 and B2 are assigned one TSC,say TSC0, while B3 and B4 have a different TSC, say TSC1. The, B1 and B2may be transmitted on TS1, and B3 and B4 may be transmitted on TS2 inthe same frame. In this way, the previous 4DL-assignment just uses twotimeslots to transmit four bursts over the air. The SAIC receiver candecode B1 and B2 with TSC0, and B3 and B4 with TSC1. Pipeline processingof decoding the four bursts may make this feature work seamlessly withconventional approaches.

Combining Timeslots

Combining one user's even number of timeslots may halve the over the air(OTA) allocation, saving battery energy. This also frees additional timefor scanning and/or monitoring of neighbor cells and system informationupdate for both serving cell and neighbor cells. There are some furtherfeatures on the network side. The network may make the additionalassignment of co-channel, co-time slot (co-TS) based on the distance ofthe new users. Initially the network may use the TCH whose users are ata similar distance. This can be done through timing TA of each user.Each user's RACH provides this info to the BS 110, 111, 114.

Changes in Network Traffic Assignment

The above also means that if two co-channel, co-TS users are moving indifferent directions one moving towards the BS 110, 111, 114 and theother moving away from the BS 110, 111, 114, there will be a point thatone of them will switch to another TCH that has a better match of thepower level. This should not be a problem, as the network may becontinuously re-allocating the users on different ARFCN and TS. Somefurther optimization may be helpful, such as optimizing selection of thenew TSC to be used, as this is related with the frequency reuse patternin the local area. One advantage of this feature is that it uses mainlysoftware changes on network side. e.g., BS 110, 111, 114 and BSC141-144. Changes on network traffic channel assignment may increase thecapacity.

Co-channel Operation for Both Voice and Data

Further improvements may be made. First, Co-TCH (co-channel andco-timeslot) may be used for voice calls as well as for data calls onthe same TCH to improve capacity-data rate. This feature may be appliedto GMSK modulated data services, such as CS1 to 4 and MCS1 to 4. 8PSK.

Fewer Timeslots Used

This feature may be applied to reuse of co-channel (co-TCH) on datacalls to achieve increased capacity. Two timeslots of data transfer maybe paired and transmitted using one timeslot with two training sequencesused in each of the corresponding bursts. They are assigned to thetarget receiver. This means that 4-timeslot downlink may be reduced to a2-timeslot downlink, which saves power and time for the receiver.Changing from 4-timeslots to 2-timeslots gives the remote station moretime to do other tasks, such as monitoring neighbor cells (NC), whichwill improve the hand off or HO.

The constraints of assignments with respect to Multi-slot Classconfiguration requirements such as Tra, Trb, Tta, Ttb—Dynamic andExtended Dynamic MAC mode rules may be relaxed. This means that thereare more choices for the network to serve the demands from variouscallers in the cell. This reduces or minimizes the number of deniedservice requests. This increases the capacity and throughput from thenetwork point of view.

Each user can use less resources without compromise of QoS. More userscan be served. In one example, this may be implemented as a softwarechange on the network side, and the remote station 123-127 is adapted toaccept additional TSCs on top of its DARP capability. The changes on thenetwork traffic channel assignment may increase the capacity-throughput.Use of uplink network resources can be conserved, even while the networkis busy. Power can be saved on the remote station 123-127. Betterhandover performance and less restriction on network assigning datacalls, and improved performance can be achieved.

Dual Carrier

The present method and apparatus may be used with dual carrier inaddition, to improve performance. For improving data rate, there is a3GPP specification which allocates dual carriers from which MS (or UE orremote station 123-127) can get two ARFCNs simultaneously in order toincrease the data rate. Thus, the remote station 123-127 uses more RFresources to get extra data throughput, which intensifies the statedissues above.

Linear GMSK Baseband

One aim of GSM voice services is to achieve the best capacity such thatall users use enough power level, but no greater, to maintain anacceptable error rate so that the user's signal may be detected. Anygreater power would add to unneeded interference experienced by otherusers. Signal quality is affected by i) the distance between the basestation 110, 111, 114 and the remote station 123-127 and ii) the RFenvironment. Therefore, different users 123-127 may be assigneddifferent power levels according to their distance and the RFenvironment. In a GSM based system, power control on the uplink anddownlink is good for limiting unnecessary interference and maintaining agood communication channel.

One advantage of using power control with a multiusers-on-one-time-slot(MUROS) enabled network is that different users 123-127 may betransmitted signals with different power levels to meet their individualneeds. A second advantage is that a non-DARP enabled remote station123-127 may be paired with a DARP enabled remote station 123-127 of thepresent method and apparatus. Then, the non-DARP capable remote station123-127 may be given a signal with a power level a few dB higher thanthe DARP enabled remote station 123-127. A third advantage is that usingpower control allows remote stations 123-127 anywhere in the cell to bepaired.

Transmitting Signals at the Same Power Level

DARP enabled mobile stations 123-127 may preferably receive signals atthe same amplitude, regardless of whether one mobile is close and theother one far away. For example, two signals transmitted by one basestation 110, 111, 114, to one mobile 123-127, the path losses for thosesignals, from the BS 110, 111, 114 to the particular mobile, say mobile123, may be the same. Similarly, the path losses for the two signalsfrom BS 110, 111, 114 to mobile 124 may be the same as each other. Thisoccurs because the signals share the same frequency and time slot.

Transmitting Signals at Different Power Levels

However, in one example, two MUROS paired remote stations 123-127 mayhave different path losses. Therefore, their signal power levels couldbe different. Hence the BS 110, 111, 114 may send MUROS signals with apower imbalance (say +10 dB to −10 dB).”

Using DARP and Non-DARP Enabled Equipment

Another feature of the present method and apparatus is the use of aMUROS signal by a legacy remote station 123-127 which does not have DARPcapability or MUROS features. The present method and apparatus allows anon-DARP remote station 123-127 to use one of two MUROS signalstransmitted on the same channel. This is achieved by ensuring that theamplitude of the signal intended for the non-DARP remote station 123-127is sufficiently greater than the amplitude of the other MUROS signal.The non-DARP remote station 123-127 does not need to indicate DARPcapability as part of its radio access capability indicating message andthe remote station 123-127 is not required to indicate a MUROSclassmark. It is desirable to pair a MUROS remote station 123-127 with alegacy remote station 123-127 in situations where such an amplitudeimbalance is acceptable or in situations where a second MUROS remotestation 123-127 cannot be identified which is suitable for pairing witha first MUROS remote station 123-127.

It follows that one reason for transmitting the two signals at differentamplitudes is to account for the situation where one of the two remotestations 123-127 are not DARP enabled, and the other is DARP enabled.The non-DARP enabled remote station 123-127 may be supplied a signalhaving more power/amplitude. (In one example, 3 to 8 dB more powerdepending on the training sequences and the corresponding degree ofinterference of the other signal (for the DARP remote station 123-127)at the non-DARP mobile station 123-127.

The range(s) of the remote stations 123-127 is a criteria for choosingremote stations 123-127 for MUROS pairing. The path loss (e.g., the RFenvironment) is another criteria used to determine the amplitudeselected for the signal transmitted to the remote station 123-127 havingthe worst path loss. This also provides the possibility of pairing alarger range (in terms of location) of remote stations 123-127 becausethe one near the BS 110, 111, 114 may be given more power than necessarypurely for an acceptable error rate, if there are no pairs which arebetter matched. An ideally matched pair of remote stations 123-127 wouldbe a pair using signals of similar amplitudes.

As stated above, it is preferable for the BS 110, 111, 114 to transmitthe two signals so that each remote station 123-127 receives its wantedsignal above its sensitivity limit. (In one example, it is at least 6 dBabove its sensitivity limit). If the non-DARP remote station 123-127 isclose to sensitivity limit, then the corresponding DARP paired remotestation 123-127 may be selected to be closer to the base station 110,111, 114 i.e., hence have less path loss, otherwise the DARP enabledremote station 123-127 may lose its signal since its signal is receivedat a lower amplitude than the amplitude of the other signal. Differentcodecs may also be used to adapt the remote stations 123-127 to enhanceperformance when a non-DARP enabled remote station 123-127 is usingMUROS enabled equipment of the present method and apparatus.

Transmitting Two Signals

Two signals may be transmitted by a base station 110, 111, 114 using oneof two approaches. (Other approaches may also be possible). In the twoalternative representations or examples, two GMSK signals may becombined with different amplitudes, A₁ for the first signal and A₂, forthe second. The ratio of amplitudes (or amplitude ratio) corresponds tothe ratio of amplitudes for the two transmitted (and received) signals.The path loss between the BS 110, 111, 114 and a given remote station123-127 is likely to be the same or near-identical for the two signalstransmitted by the BS 110, 111, 114.

As discussed above, the BS 110, 111, 114 transmits the signals atsuitable amplitudes so that each remote station 123-127 of the presentmethod and apparatus receives its own signal at a high enough amplitudeand two signals have an amplitude ratio such that the two signalscorresponding to the two TSCs may be detected. The signals may be bothtransmitted by one transmitter of a base station 110, 111, 114 on thesame channel (comprising only one timeslot and only one frequency) withboth signals received by the receiver of a first remote station 123-127in the amplitude ratio and both signals received by the receiver of asecond remote station 123-127 in the same amplitude ratio.

The ratio of amplitudes can be expressed as the product of A₂ divided byA₁ or the product of A₁ divided by A₂ The ratio is expressed in decibelsas 20*log 10(A₂/A₁) or 20*log 10(A₁/A₂). The ratio can be adjusted andpreferentially has a magnitude of either substantially 0 dB orsubstantially between 8 dB and 10 dB. The ratio can be less than one orgreater than one and hence, the ratio expressed in dB can becorrespondingly positive or negative.

In a first approach or example, steps can be carried out in accordancewith the flow diagram shown in FIG. 21A. The two signals may be GMSKmodulated (step 2110) and added together (step 2140), each with arespective power level chosen to offset attenuation due to the differentsignal distances and environments. That is, each signal is multiplied bya its own gain (step 2130). The gains may be chosen to be in the ratioR=A2/A1, which yields the correct amplitude (hence power) ratio for thetwo signals. This is what yields the 8-10 dB ratio discussed above.

If both remote stations are DARP enabled, it is preferred in one examplefor the ratio to be unity (0 dB). For one remote station 123-127 to beDARP enabled and the other non-DARP enabled, it is preferred in oneexample for the ratio to be 8-10 dB in favour of the non-DARP enabledremote station 123-127. This may be referred to as differential powercontrol and it may be implemented either at baseband or at RF, or both.Further (common) power control can be applied to both signals equally(to account for range, path loss of the remote station 123-127 requiringhighest amplitude (e.g. the remote station 123-127 may be further away).

This additional power control may be applied partly at baseband andpartly at RF, or only at RF. At baseband, common power control isapplied to both signals by the equal scaling of gains A1 and A2, e.g.multiplying them both by 1.5. Common power control at RF is normallyexecuted in the power amplifier (PA) 1830. It could also be partlyexecuted in the RF modulator 1825.

Also, one of the signals may be phase shifted by π/2 relative to theother signal The π/2 phase shift is shown as step 2120 of FIG. 21A, inblock 1810 of FIGS. 15, 16, and 19, and in blocks 1818 and 1819 of FIGS.17 and 18. The added signals are then transmitted (step 2150). Anexample apparatus is shown in FIG. 15. Preferably, one of the twosignals is shifted in phase relative to the other signal prior totransmission, and preferably by 90 degrees, i.e., π/2 radians. Howeverthe present method and apparatus may work with any phase shift betweenthe signals including zero phase shift. If more than two signals aretransmitted, each signal can be offset in phase from the others. Forexample, for three signals each can be offset from the others by 120degrees. In FIG. 21A, the steps of phase shifting and amplifying by again may be done in either order as illustrated where steps 2120 and2130 are reversed in the flowchart of FIG. 21C compared to FIG. 21A.

FIG. 15 illustrates an apparatus to combine two signals. It comprisestwo GMSK baseband modulators 1805 having at least one input and at leastone output, whereby the signals are modulated. One amplifier 1815 isconnected in series with each GMSK modulator 1805, whereby the twosignals are multiplied by a respective amplitude, A₁ for the firstsignal and A₂, for the second signal where A1 is equal to cos α and A2is equal to sin α.

The output of each amplifier 1815 is combined in a combiner (adder)1820, and a phase shifter 1810 is preferably operably connected betweenone of the series combinations of baseband modulator 1805 and amplifier1815, so that one of said signals is phase shifted with respect to theother signal. The output of the combiner 1820 is input into a RFmodulator/power amplifier module 1823, whereby the combined signals areRF modulated and transmitted. By RF modulated, it is meant that thesignals are upconverted from baseband to RF frequency. It is noted thatthe phase shifter 1810 may be operably connected between one amplifier1815 and the combiner 1820.

FIG. 16, FIG. 17 and FIG. 18 illustrate respectively second, third andfourth examples of apparatus for combining and transmitting two signalswith different amplitudes.

In FIG. 16, the RF modulator & power amplifier 1823 is represented by aseries connection of a RF modulator 1825 and power amplifier 1830. Theexample shown in FIG. 17 shows the use of GMSK baseband modulators 1805and one RF modulator 1862. The first and second data are basebandmodulated by baseband modulators 1805. Baseband modulators 1805 eachcomprise a differential encoder, an integrator and a Gaussian lowpassfilter 1811. The outputs of the respective baseband modulators 1805 areeach a digital value representing the phase of the GMSK modulated signal(φ(t) for the first signal and φ′(t) for the second signal). Block 1816comprises a function which produces the cosine of said phase of thefirst signal and multiples the cosine by a gain A1 to provide an outputsignal, A1 cos φ(t) at the output of the block 1816.

Block 1818 comprises a function which adds a phase shift of pi/2 radians(90 degrees) to the phase of the second signal, produces the cosine ofthe resulting phase and multiples the cosine by a gain A2 to provide anoutput signal, A2 cos(φ′(t)+90) at the output of the block 1818.

Block 1817 comprises a function which produces the sine of said phase ofthe first signal and multiples the sine by a gain A1 to provide anoutput signal, A1 sin φ(t) at the output of the block 1817.

Block 1819 comprises a function which adds a phase shift of pi/2 radians(90 degrees) to the phase of the second signal, produces the sine of theresulting phase and multiples the sine by a gain A2 to provide an outputsignal, A2 sin (φ′(t)+90) at the output of the block 1819.

The outputs of blocks 1816 and 1818 are summed/combined by combiner 1807to produce a summed I (in-phase) GMSK modulated baseband signal. Theoutputs of blocks 1817 and 1819 are summed/combined by combiner 1827 toproduce a summed Q (quadrature-phase) GMSK modulated baseband signal.

Preferably, as shown, all operations and signals in blocks 1816-1819,1807 and 1827, are digital, and so the outputs of the combiners 1807,1827 are also digital values. Alternatively, some of the functions couldbe performed by analogue circuitry by the use of digital-to-analogueconversion, etc.

The summed GMSK modulated baseband digital signals output from combiners1807, 1827 are each input to a digital-to-analogue converter (DAC orD/A) 1850, 1852 and suitably lowpass filtered (filter not shown) to formI and Q inputs to the RF modulator 1862, which upconverts the basebandsignals onto a carrier frequency, the carrier frequency provided bylocal oscillator 421, to form a transmitted signal.

FIG. 17 and FIG. 18 each illustrate two GMSK baseband modulators 1805,each comprising a differential encoder 1807, an integrator 1809 operablyconnected to said differential encoder 1807, and a Gaussian low passfilter 1811 operably connected to said integrator 1809 The example shownin FIG. 18 shows the use of two GMSK baseband modulators 1805 and two RFmodulators 1862, 1864. The output of each RF modulator 1862, 1864, oneRF modulator 1862, 1864 for each of the first and second datarespectively, are summed/combined with each other, in combiner 1828, fortransmission.

FIG. 19 illustrates an apparatus for combining two signals by mappingtwo users' data onto the I and Q axis respectively of a QPSKconstellation. According to this approach, the data of users 1 and 2 ismapped to the I and Q axis respectively of the QPSK constellation, withπ/2 progressive phase rotation on every symbol (like EGPRS 3π/8 rotationon every symbol, but with pi/2 instead of 3π/8) with each user's signalpower level determined by the A₁ and A₂ gains Amplifier gain for the Isignal (for user 1) is A₁ which is equal to the cosine of alpha, α.Amplifier gain for the Q signal is A₂ which is equal to the sine ofalpha. Alpha is the angle whose tangent is the amplitude ratio betweenthe two signals for user 1 and user 2 respectively.

The baseband modulators 1805 comprise a binary phase shift keying (BPSK)baseband modulator 18051 for a first signal represented on an I axis anda BPSK baseband modulator 18052 for a second signal represented on a Qaxis. The transmit I and Q signals which are input to phase rotators18201 and 18202 of FIG. 19, may be filtered before or after phaseshifting, by means of linear Gaussian filter or pulse-shaping filters18211, 18212 (e.g., for use with EGPRS 8PSK modulation) to satisfy theGSM spectrum mask criteria. FIG. 19 shows a suitable pulse-shapingfilter 18211 operably connected between said phase rotator 18201 and aRF modulator/power amplifier 1823 and a suitable pulse-shaping filter18212 operably connected between said phase rotator 18202 and the RFmodulator/power amplifier 1823. The RF modulator and PA 1823 acts to RFmodulate, combine and amplify the I and Q signals to provide a combinedsignal for transmission via the antenna 1850.

In FIGS. 17 and 18 a −π/2 phase shift is introduced to the localoscillator signal by the outputs of the splitter 1812. Thus, the LO issplit into in-phase and quadrature-phase carrier signals and input toeach of two mixers/multipliers 1840-1844, 1848.

FIG. 20 is a QPSK constellation diagram for a signal output from theapparatus shown in any of FIGS. 17 to 19.

The steps executed in the two approaches (GMSK or QPSK based) areillustrated in the flowcharts of FIGS. 21A and 21B respectively. In FIG.21A, the steps of phase shifting and amplifying by a gain may be done ineither order as illustrated in FIG. 21C where the order of steps 2120and 2130 are reversed compared to the flowchart of FIG. 21A.

With both of the approaches, when a MUROS enabled BS 110, 111, 114 sendsa RF burst on the downlink channel, the BS 110, 111, 114 controls twoparameters:

First, the I and Q data streams are normalized, which enhances theresolution and dynamic range of the digital-to-analog controller (DAC)1850, 1852 used.

Second, the power level used for the signal burst containing both the Iand Q signals is controlled. This is used to determine the gain for thepower amplifier (PA) (see below).

The following are additional steps which may be taken by a MUROS-enabledbase station, as compared to a legacy base station, to use the presentmethod and apparatus. See FIG. 22 for a simplified flow diagram.

First, use the path loss of the two signals to derive the power level tobe used for both co-TCH callers, say power level 1, P1, for user 1 andpower level 2, P2, for user 2 respectively (In this example, the powerlevel is expressed in Watts, not dBm) (step 2210)

Second, calculate the amplitude ratio R of the two power levels (step2220):R=√{square root over (VP2/P1)}

Third, determine the gains, G1 and G2, for the two users or callers,user 1 and user 2 respectively (step 2230): In one example, for user 1,G1=A₁=cos(α), and for user 2, G2=A₂=sin(α), while α=arc tangent (R).Also, A₂/A₁=sin(α)/cos(α)=tan(α)=R.

Fourth, determine the gain for the power amplifier by considering thepower level:P=P1+P2.  (step 2240)

The present method and apparatus combines two signals that may havedifferent phases and power levels, so that: 1) Each user may receive awanted signal having the required amplitude together with an unwantedsignal, such that the amplitude of the unwanted signal is less than theamplitude at which the unwanted signal would cause unacceptableinterference to the wanted signal. This may avoid excessive amplitudethat could interfere with others in another cell.

However, in some cases, a lower power remote station 123-127 (lowerpower is used because it is nearer to the base station 110, 111, 114)can have higher power instead (more than the remote station 123-127needs) in order to pair with a remote station 123-127 that is furtheraway from the base station 110, 111, 114). The zero-crossing of themodulation ‘eye diagram’ may be avoided, which may help avoid AM-PMconversion distortion and low signal-to-noise ratio (SNR). In addition,a legacy (non-MUROS) remote station, either non-DARP enabled or DARPenabled, may be used with the MUROS enabled network, i.e., base station110, 111, 114 or base station controller 140-143.

Adaptive Pulse Shaping

The following passage describes a situation in which two receivers eachreceive the same two co-channel signals. The principles described applyequally well to a situation in which there are more than two (forexample N) receivers, each receiving the same N co-channel signals. Atransmitter transmits first and second signals for first and secondreceivers respectively, each receiver receiving both signals atdifferent amplitudes on the same channel.

There is a conflict between the requirements of the first and secondreceiver. Each receiver operates best if it receives its own signal atamplitude (or power level) higher than for the other signal. If thetransmitter increases the amplitude of the signal for the secondreceiver, the second signal acts more as interference for the firstreceiver. Conversely, if the transmitter increases the amplitude of thesignal for the first receiver, the first signal acts more asinterference for the second receiver.

If the first receiver is closer to the transmitter than the secondreceiver, or the second receiver suffers greater path loss between thereceiver and the transmitter than for the first receiver, then thetransmitter may transmit the second signal (for the second receiver)with higher amplitude and the first signal with lower amplitude. Thesecond receiver then receives its own signal at a higher amplitude thanthat of the first receiver's signal, for example 10 dB or 12 dB higher.This is particularly advantageous for the second receiver if the secondreceiver receives its signal at an amplitude which is close to theminimum for which the receiver can make use of the signal (thesensitivity limit).

The first receiver, on the other hand, receives its own first signal ata power level less than the level at which it receives the second,co-channel signal.

This situation may occur for example if the second receiver is near acell boundary of a large cell, or in an area having large signalattenuation, and the first receiver is much closer to the transmitterthan the second receiver. Such a situation is sometimes referred to asthe ‘near-far problem’.

The first receiver may be able to tolerate the co-channel second signalat a high amplitude relative to the amplitude of its own signal. Forexample, the first receiver may receive its own signal at the sameamplitude as for the second signal. This improved tolerance may be dueto the receiver having improved capability such as DARP. The firstreceiver which receives its own first signal at amplitude less than theamplitude of the second signal, cannot make use of its signal if thepower of the second signal is greater than a given threshold, relativeto the power of the first signal. This places a limit on the maximumrange of a second receiver which needs to receive its own second signalat significantly higher amplitude than the amplitude at which itreceives the first signal.

One way of addressing this limitation is to increase the effective powerof the first signal by spreading its bandwidth by means of pulse-shapingusing a broader-bandwidth baseband filter. This allows the power levelof the second signal to be increased, while still allowing the firstreceiver to receive and make use of the first signal. Additionally, dueto the broader bandwidth and associated narrower pulse-shaping of thefirst signal, the second receiver can operate effectively whilereceiving a higher power level of interfering first signal because thepower of the interfering first signal is spread across a wider bandwidthan therefore the first signal acts less as interference than if thefirst signal occupied a narrower bandwidth.

FIG. 24 is a diagram of a transmitting apparatus 240. A signal source241 provides an input signal to a low-pass filter 242 which filters theinput signal to produce a filtered signal. The filter 242 is selectivelyoperable in a first mode in which the input signal is filtered within anarrower bandwidth; and in a second mode in which the input signal isfiltered within a broader bandwidth. The filtered signal is output to amodulator 243 which modulates the filtered signal to produce a modulatedsignal. The modulated signal is output to a transmitter 244 whichtransmits the modulated signal.

A controller 245, connected to the filter 242 and to the transmitter,controls operation of the filter and transmitter depending on therequired amplitude of the transmitted signal. If the required amplitudeof transmitted signal is higher, the filter is configured to filter theinput signal in the first mode and the transmitter is configured totransmit the transmitted signal at a higher power level. If the requiredamplitude of the transmitted signal is lower, the filter is configuredto filter the input signal in the second mode and the transmitter isconfigured to transmit the transmitted signal at a lower power level.

FIG. 25 is a diagram of a transmitting apparatus 250 for transmittingtwo signals substantially simultaneously. The apparatus 250 shown inFIG. 25 is similar to the apparatus 240 shown in FIG. 24. The apparatus250 comprises two signal sources 251, 256, two filters 252, 257, twomodulators 253, 258 and two transmitters 254, 259, controlled by acontroller 255. The apparatus comprises two signal paths 251 to 254 and256 to 259 along which respective input signals travel. Thus, signalsource 251, low-pass filter 252, modulator 253 and transmitter 254cooperate to produce a first transmitted signal. Similarly, signalsource 256, low-pass filter 257, modulator 258 and transmitter 259cooperate to produce a second transmitted signal.

FIG. 26 is a diagram of a transmitting apparatus 260 for transmittingtwo signals in combination. The apparatus 260 shown in FIG. 26 issimilar to the apparatus 250 shown in FIG. 25 in that it comprises twosignal paths 261 to 265 and 266 to 268 along which respective inputsignals travel. The apparatus 260 also comprises: a combiner 269 betweenthe two modulators 263, 268; and a transmitter 270 for combining thesignals from the two paths 261 to 263 and 266 to 268 beforetransmission. The combiner 269 combines the first and second modulatedsignals. The combiner may comprise for example a passive or activesumming network. The combiner outputs a combined signal to a transmitter270. The transmitter 270 transmits the combined signal.

FIG. 27 is a flow diagram representing a method for filtering,modulating and transmitting a signal according to a first or second modeof operation. At the start of the flowchart, a first input signal isprovided 272. A decision is made in block 273 whether to carry out themethod according to a first or a second mode of operation. The decisionis based on the required amplitude for the transmitted signal.

If the required amplitude is higher then according to the first mode inblock 274 the input signal is low-pass filtered within a narrowerbandwidth, then in block 275 the low-pass signal is modulated to producea modulated signal, then in block 276 the modulated signal istransmitted at a higher amplitude.

If the required amplitude is lower then in block 278 the input signal islow-pass filtered within a broader bandwidth, then in block 279 thelow-pass signal is modulated to produce a modulated signal, then inblock 280 the modulated signal is transmitted at a lower amplitude.

FIG. 28 is a diagram of a method for filtering, modulating, combiningand transmitting two signals according to a first and second mode ofoperation respectively.

A first input signal is provided in block 282. The input signal islow-pass filtered in block 283 to produce a first low-pass filteredsignal. The low-pass filtered signal is modulated in block 284 toproduce a first modulated signal. A second input signal is provided inblock 285. The second input signal is low-pass filtered in block 286 toproduce a second low-pass filtered signal. The second low-pass filteredsignal is modulated in block 287 to produce a second modulated signal.The first and second modulated signals are combined in block 288 toproduce a combined signal. The combined signal is transmitted in block289. The transmitted signal output from block 289 comprises first andsecond transmitted signals, for first and second input signalsrespectively. The first and second input signals are provided to blocks282 and 283 respectively according to the relative required amplitudesfor the first and second transmitted signals respectively.

Because the required amplitude of the first transmitted signal is higherthan the required amplitude of the second transmitted signal, the firstinput signal is input to block 282 as shown.

As an alternative variation of this method (not illustrated in theaccompanying drawings), the two modulated signals may each betransmitted separately and not combined. The combining operation 288would thus not be performed and the modulated signals would betransmitted directly.

FIG. 29 is a diagram of a transmitting apparatus 290 for combining andtransmitting two signals to produce a QPSK-modulated, phase-rotatedsignal. A first input signal comprising a first sequence of symbols isinput to a first BPSK modulator 291. The modulator 291 is configured toBPSK modulate the first input signal to produce a first BPSK-modulatedsignal. A first phase rotator 292, coupled to the first BPSK modulator291, is configured to increment the phase of the first BPSK-modulatedsignal by a prescribed phase increment, in this example 90 degrees (pi/2radians), on every symbol of the first sequence of symbols, to produce afirst phase-shifted signal. A first amplifier 293, coupled to the firstphase rotator 292, is configured to amplify the first phase-shiftedsignal by a first prescribed gain to produce a first amplified signal.In this example, the gain is shown as A, which is equal to the cosine ofalpha (α).

A first pulse-shaping filter 297, coupled to the first amplifier 293, isconfigured to low-pass filter the first amplified signal to produce afirst filtered baseband signal.

A second input signal comprising a second sequence of symbols is inputto a second BPSK modulator 294. The modulator 294 is configured to BPSKmodulate the second input signal to produce a second BPSK-modulatedsignal. A second phase rotator 295, coupled to the second BPSK modulator294, is configured to increment the phase of the second BPSK-modulatedsignal by a prescribed phase increment, in this example 90 degrees (pi/2radians), on every symbol of the second sequence of symbols, to producea second phase-shifted signal.

A second amplifier 296, coupled to the second phase rotator 295, isconfigured to amplify the second phase-shifted signal by a secondprescribed gain to produce a second amplified signal. In this example,the gain is shown as B, which is equal to the sine of alpha (α).

A second pulse-shaping filter 298, coupled to the second amplifier 296,is configured to low-pass filter the second amplified signal to producea second filtered baseband signal.

A radio frequency (RF) transmitter circuit 291, coupled to the first andsecond pulse shaping filters 297, 298 is configured to modulate,combine, amplify and transmit the first and second filtered basebandsignals to produce a combined, QPSK-modulated signal. In this example,the transmitter circuit 291 quadrature modulates the first and secondfiltered baseband signals so that the first filtered baseband signalforms an in-phase input, and the second filtered baseband signal forms aquadrature input, of an I-Q modulator.

The operations of the elements 291 to 2911 shown in the figure arecontrolled by a microprocessor (not shown) which is coupled to a solidstate memory (not shown). The microprocessor is operable to control theelements according to instructions stored in the memory.

FIG. 30 is a graph 300 showing a broader pulse 301 and a narrower pulse302. The broader pulse 301 corresponds to the first mode (using thefirst filter) and the narrower pulse corresponds to the second mode(using the second filter). The horizontal axis of the graph representsunits of time. The broader pulse 301 represents the time envelope of thefirst filtered signal and the narrower pulse represents the timeenvelope of the second filtered signal. The broader pulse 301 is outputby the first filter which in this example is a linear Gaussian filter(LGF) and the narrower pulse is output by the second filter which inthis example is a root raised cosine (RRC) filter.

As shown in FIG. 29, the first filtered signal is output from the firstpulse shaping filter 297, and the second filtered signal is output fromthe second pulse shaping filter 299. The RRC filter is used to narrowthe pulse width of the second signal (having a lower power level) in thetime domain so that the frequency spectrum is correspondingly broader.When the second signal is added to the first signal the frequencyspectral envelope of the combined first and second signal fits withinthe predefined spectral mask. Thus, the second signal, when transmitted,will contain greater power, which improves its performance.

FIG. 31 is a graph 310 showing test results. A spectral mask 311 isshown as a solid line comprising three straight line portions. A firstspectral envelope 312 is a curve representing the measured spectralenvelope of a first transmitted signal corresponding to the firstfiltered signal produced using a LGF filter. A second spectral envelope313 is the spectral envelope of a second transmitted signalcorresponding to the second filtered signal. The second filtered signalis produced using a RRC filter which has broader bandwidth than the LGFfilter.

The first transmitted signal has a power level which is 5 dB higher thanthat of the second transmitted signal. The spectral envelope of eachsignal is relative to the amplitude of the signal. The spectralenvelopes have values of power spectrum magnitude, in units of dB,across a range of frequencies from zero to around 330 kHz. The graphshows that the spectral envelopes of both the first and secondtransmitted signal are within (i.e. below) the spectral mask 311 for allfrequencies shown.

FIG. 32 is a graph showing curves 321 to 324 which represent theperformances of four receivers. Curves 321 to 324 are each a set ofstraight line portions which join points representing values of frameerror rate (FER) expressed as a ratio, each value being for a givencarrier-to-interference ratio (C/I) expressed as a ratio in decibels(dB). Two of the receivers have enhanced capability known as DownlinkAdvanced Receiver Performance (DARP), i.e. they are DARP receivers. Theother two receivers do not have this enhanced capability, i.e. they arenon-DARP receivers. Curves 321 and 322 each correspond to a DARPreceiver which receives a signal having a lower amplitude. Curves 323and 324 each correspond to a non-DARP receiver which receives a signalhaving a higher amplitude.

Curves 321 and 323 each correspond to a receiver having a LGF pulseshaping filter. Curves 322 and 324 each correspond to a receiver havinga RRC pulse shaping filter.

The results show that the use of the RRC filter (having a widerbandwidth) for producing the second (weaker) filtered signal gives asignificant improvement in the FER for a given C/I, for the receiverreceiving the second signal at a lower amplitude (see curves 321, 322).The results also show, surprisingly, that the use of the RRC filter forthe first (stronger) signal also significantly improves the performanceof the receiver receiving the first signal at a higher amplitude (seecurves 323, 324). The RRC filter suppresses intersymbol interference(ISI) and minimizes the interference to the stronger signal, thusimproving performance of the stronger signal as well.

It follows that, for a given FER, a lower amplitude signal can betransmitted for the second receiver and/or a higher amplitude can betransmitted for the first receiver, than would be the case if only LGFpulse shaping filters are used.

This method may be stored as executable instructions in software storedin memory 962 which are executed by processor 960 in the BTS as shown inFIG. 23. It may also be stored as executable instructions in softwarestored in memory which are executed by a processor in the BSC 140-143.The remote station 123-127 uses the TSC it is instructed to use.

Narrowing the pulse in the time domain (and correspondingly making thebandwidth of the spectral envelope broader) is more efficient thanretransmission of lost data, because time is not spent resending data.

A variable pulse shape can resolve fundamental trade-offs (such as (a)good power level on one channel vs. interference it may introduce to thenearby channel; and (b) a good throughput for one user vs capacitywithin a certain area, etc). Narrowing the pulse in the time domain mayincrease the power of the signal for a particular user, for example whenthe link conditions are not favourable, e.g. large path loss or badchannel distortion due to multipath.

The pulse shaping adaptation means that a particular technology (e.g.GSM or WCDMA) with each modulation scheme (e.g. GMSK and 8PSK) may havemore than one pulse shaping to adapt to different situations. (CurrentlyGMSK and 8PSK each have fixed pulse shaping and adaptation of pulseshaping may not be possible without change to the technology standards).The pulse shape can be broader or narrower to suit the situation (e.g.8PSK can have more than one pulse shaping, and so can GMSK). Theselection of different pulse shaping may be based on a feedbackmechanism.

For example the transmitter (e.g. BS 110, 111, 114) can use differentpulse shaping based on the transmitter receiving feedbacksignals/indications from the receiver (e.g. remote terminal 123-127) fordifferent pulse shapes that are used. In this way the best pulse shapingfor a particular situation may be used to get the best performance ofthe communication system. The feedback mechanism can be used for eithera single transmitter-receiver pair or for several receivers (remotestations) with one transmitter. The feedback mechanism may thus improvethe performance of the link either for an individual pair of onereceiver and one transmitter or for multiple transmitter-receiver pairsin combination, e.g. a portion of a communication system (e.g. a cell, asite, a town or a city).

Pulse shaping adaptation as described above could have advantages asdescribed below.

1. Adapting with Spectrum Usage

One way of defining the pulse shape is to fix the spectrum mask anddesign a pulse shape to ensure the spectral envelope falls within thatmask. Non-linear elements of the transmitter e.g. the power amplifier(PA) 1830, 1823, in addition to narrow pulse width, can contribute tospectral re-growth whereby fluctuations in amplitude of the transmittedsignal give rise to increased amplitude of the signal's spectralenvelope due to AM-PM conversion in the PA. In modern devices, forexample, the PA is more linear. The original mask allowed for margin dueto e.g. non-linearity of older PAs. With pulse shaping adaption, themargin may be used to broaden the spectrum of the transmitted signal.

The communication system may be used for different amounts of traffic atany time. For example, there may be few users, or there may be manyusers in a cell or sector aa. The adaptive pulse shaping can makeadaptations according to the amount of traffic. For example when thereare a few users on one channel within a cell (a sparse user case whichhas no frequency reuse), the pulse shaping can be broader, yet minimalinterference will be introduced between the signals for different users.When there are many users in the same area (a dense user case which hassignificant frequency reuse), the pulse shaping can be narrower tominimise the interference between the users' signals. Each case can beassessed and adapted by the feedback loop of the system, large or small.

2. Adapting with Type of Pulse Shaping to Reduce ISI

A family of RRC pulse shaping can put together with the GMSK and 8PSK sothat when the conditions are right, they can both adapt to RRC pulseshaping to reduce ISI.

3. Adaption from Legacy Non-linear to Modern Linear Modulation

GMSK was originally chosen as the default GSM modulation because GMSK,being a constant envelope non-linear modulation, can make use of a lowcost, non-linear PA, while achieving greater power efficiency. Howeverfrom the performance point of view (e.g. non-linearity and inter-symbolinterference or ISI), GMSK may not be as good as a linear modulationscheme such as BPSK or QPSK with the linear Gaussian pulse shaping. Withthe availability of linear PAs, other modulations beside GMSK may beused. For better performance, BPSK with a family of RRC pulse shapingmay be used as an alternative to GMSK. It is worth noting that a furtheradvantage may be achieved if the receiver can also be arranged to have afilter corresponding to that used in the transmitter. However withoutchanging the filter of the receiver, the benefit is still significant.

In one example, the pulse shaping may be implemented as a finite impulseresponse (FIR) filter, for which an array holding the values of thepulse shaping is convolved with the symbols. The adaptive pulse shapingcomprises multiple arrays that hold a family of pulse shapes. The arrayscan be put into a linear memory and the pulse shapes' arrays can beaccessed with an indexed pointer. The feedback mechanism provides theindex, and the adaptive pointer using such an index will then point tothe right pulse data array to be used. The penalty is larger memory forholding extra arrays of pulse shapes. The pulse shaping filters can be afamily of RRC filters that have different properties, such as bandwidth.

FIG. 33 comprises a selection of narrower and broader pulses whichresult from using RRC filters having different bandwidths. Thehorizontal axis represents linear time. The vertical axis represents theamplitude of a data pulse at the output of the filter. The pulselabelled 3301 is narrowest in the time domain as shown in the figure.Therefore pulse 3301 has the broadest bandwidth. The pulse labelled 3302has a bandwidth less than the bandwidth of pulse 3301, and so on. Pulse3304 has the broadest bandwidth,

It is better to include the existing pulse shaping in the family of thepulse shaping series, so that the series can start from this defaultpulse shaping, while adaptation can move to a better one when conditionsallow. In this way, the adaptation would be no worse than the currentperformance as the default case is the baseline for adaptation. Thishelps the modulation to evolve to a better method with the control ofthe network. The simulation shown in FIG. 33 indicates that the a coupleof dB gain may be obtained from this change.

Signaling

Because the signaling channel has good coding and forward errorcorrection (FEC) capability, it only requires a minimal signal qualityto detect a signal. Any higher signal power levels than that would wastepower and create interference to other remote stations 123-127. In thisway each communication will drop the power level to minimizeinterference to another remote station 123-127 in the network, whilemaintaining an acceptable BER which may processed by FEC to allowdetection of the desired signal.

Benefits of the present method and apparatus include: avoiding excessinterference in the network between signals for different users; andallowing the network to support potential increased capacity and savingbattery life and prolong the talk time and standby time.

Vamos Uplink Modulation Adaptation

Two signals intended for two VAMOS remote stations can have a powerimbalance due to their path loss differences as discussed above in thesection titled “Adaptive Pulse Shaping.” When that happens pulse shapingadaptation can be employed and can improve performance. Pulse shapingadaptation can also be used on the uplink to get similar benefit,although it might be difficult to get as much benefit from use of pulseshaping adaptation on the uplink as on the downlink since the two localoscillators (LO's) of the two remote stations are independent. Also, aseach remote station in VAMOS mode doesn't know the power level of theother remote station, it may not be possible and or suitable for theremote stations (or handsets) to make a decision whether to employ pulseshaping adaptation on the uplink.

On the downlink pulse shaping adaptation can be implemented in arelatively straight forward manner as the BTS 110, 111, 114 has bothremote stations' 123-127 reported data (e.g. received data qualityindicator) and the base station transmits data for both remote stationsat known power levels controlled by the base station. Therefore the basestation can decide the power imbalance factor. However, this is not thecase for the uplink since each remote station 123-127 is independent ofeach other and doesn't have any knowledge of the other co-TCH user'sdata and path loss to the base station 110, 111, 114. Therefore, it isnot as straight forward to address the power imbalance by means of pulseshaping adaptation on the uplink. Methods and apparatus for implementingpulse shaping adaptation on the uplink will now be described, wherein abase station determines pulse shape adaptation at a remote station.

In one example, a power imbalance is addressed when a BS 110, 111, 114uses control channel data (SACCH) messages to indicate to a VAMOScapable remote station 123-127 the power imbalance. There are three bitsthat are available to use in the SACCH for this function.

The SACCH's normal function is to carry system information messages onthe downlink, carry receiver measurement reports on the uplink and toperform closed-loop power and timing control. Actual power and timingadvance settings currently in use are transmitted on the uplink. Ordered(i.e. prescribed or selected) power and timing values are transmitted onthe downlink.

The SACCH block structure defined in 3GPP TS 44.004 section 7.1.1 isshown in FIG. 33. The 23 parts of SACCH blocks are used in the downlinkin the following way. The ordered remote station (or MS) 123-127 powerlevel is carried in the first 5 bits of part 1. The ordered timingadvance is carried in the 8 bits of part 2.

In the case of the GSM 400 band, the extended timing advance informationelement (IE) is supported and the maximum timing advance value TA_(max)is specified to be 219. This is the only case in which ordered timingadvance uses all eight bits of the ordered timing advance data bits(part 2), so as to allow for a cell size or range of up to 120 km.Otherwise, for a non-GSM 400 band, the ordered timing advance is a G-bitlong information element to cover cell size or range up to 35 km(distance between remote station to BTS).

Changes to the existing SACCH can be made to implement the uplink VAMOSmodulation adaptation method of the present patent application. In oneexample, Part 1 and part 2 are modified as shown in FIG. 34. As before,the ordered remote station (or MS) 123-127 power level is carried in thefirst 5 bits of part 1. However, bit 8 of part 1 carries a pulse shapingfunction (PSF) flag which indicates whether or not the BS 110, 111, 114wants VAMOS PSF adaption on the uplink. The ordered timing advance hasalso changed. It is now carried in the first 6 bits of part 2. Bits 7and 8 of part 2 designate the PSF type, i.e., RRC filter selection.

Thus, the present method and apparatus modifies two fields of the SACCHto enable pulse shaping filtering. First, the spare bit of part 1 isused to carry the PSF, the pulse shaping function flag, which indicateswhether or not the BS 110, 111, 114 wants VAMOS PSF adaptation on theuplink. PSF=0 means no VAMOS PSF adaptation.

It is unlikely that, at a distance further than 35 km away from a BS110, 111, 114 there is a dense population of remote stations that maycause voice service capacity issues. Therefore, using VAMOS and anextended timing advance may not be a major concern at this distance, anda remote station 123-127 may transmit data using the default modulationin the VAMOS mode using a LGF or standard GMSK. However, for a distanceof 35 km or less, pulse shaping adaptation may be used for the uplink,for either a GSM 400 system or a non-GSM 400 system.

For a GSM 400 system, the timing advance field is 8 bits long and nospare bit may be used for the PSF type. However, the spare bit of part 1may still be used to help the case of the GSM 400 MHz band. The remotestation 123-127 may use all 8 bits of part 2 for the timing advancevalue for large cell operation. Thus, there won't be a 2-bit PSF typefield in part 2. However, the 1-bit field in part 1 may be used. Whenthe 1-bit field in part 1, the PSF flag, equals 1 VAMOS PSF adaptationon the uplink is on. Also, the 6-bit timing advance value indicatesnormal size cell operation. In addition, with the GSM 400 band or anyband, the LGF or GMSK filtering may be used as the default pulse shapingas shown in Table 4. For the GSM 400 band, only one pulse shaping otherthan the default pulse shaping is provisioned for because there are noavailable bits to indicate a pulse shaping filter type to use.

Bits 7 and 8 of part 2 (part of ordered timing advance) are used toindicate PSF type, the index of pulse shapes, e.g., which RRC filtershape/type to use. This would only be effective or applicable when thepulse shaping function flag, PSF, =1.

The bits 7 and 8 describes RRC filter selections, such as

TABLE 4 Bits Example of 8, 7 PSF type details for the adaptation 00 LGFor GMSK Default pulse shaping 01 RRC-1 230 kHz bandwidth 10 RRC-2 250kHz bandwidth 11 RRC-3 270 kHz bandwidth

For the uplink example above, four values of PSF may be enough toprovide a desired improvement in headroom. Therefore, a 2-bit field isjust the right size (2²=4) to carry the PSF information. Of course, inother examples, a different size field may be used. In addition, the PSFfield is located in a good location within the downlink SACCH.

The proposed changes to the fields of the SACCH may not affect legacyremote stations 123-127 because the modified fields are not intended tobe used by legacy remote stations 123-127.

The uplink modulation adaptation method and apparatus of the presentpatent application may be used to improve the Frame Error Rateperformance in VAMOS mode. It makes use of the existing spare bits inthe SAACH to enhance the modulation adaptation of the uplink. As aresult, a voice call may be more reliable and have improved voicequality.

FIG. 35 is a flowchart illustrating the steps taken to modify fields ofthe SACCH to enable pulse shaping filtering in a GSM system. In a firststep, a decision is made on whether there is a power imbalance that maybe corrected (Step 802). If the answer is yes, then it is determinedwhether the system is a GSM 400 band system (Step 804). If the answer isyes, determine (in block 810) whether the range or distance between basestation and remote station is greater than a threshold range. If thedetermination is that the range is above the threshold range, modify afirst part of a SACCH to carry a pulse shaping function flag and use LGFor GMSK as default pulse shaping (Step 806). If the answer is no, modifya first part of a SACCH to carry a pulse shaping function flag andmodify a second part to carry a pulse shaping type field. (Step 808).

If the determination in block 804 is no, i.e. the system is not a GSM400 band system, then (in block 808) modify a first part of a SACCH tocarry a pulse shaping function flag and modify a second part to carry apulse shaping type field. Optionally, the block 810 may be omitted suchthat, if the determination of block 804 is yes, then the determinationof block 806 is carried out directly.

This method may be stored as executable instructions in software storedin memory 962 which are executed by processor 960 in the BTS as shown inFIG. 23.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted as one or more instructions or code on a computer-readablemedium. Computer-readable media includes both computer storage media andcommunication media including any medium that facilitates transfer of acomputer program from one place to another. A storage media may be anyavailable media that can be accessed by a general purpose or specialpurpose computer. By way of example, and not limitation, suchcomputer-readable media can comprise RAM, ROM, EEPROM, CD-ROM, DVD,HD-DVD, Blu-ray or other optical disk storage, magnetic disk storage orother magnetic storage devices, or any other medium that can be used tocarry or store desired program code means in the form of instructions ordata structures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The methods described herein may be implemented by various means. Forexample, these methods may be implemented in hardware, firmware,software, or a combination thereof. For a hardware implementation, theprocessing units used to detect for ACI, filter the I and Q samples,cancel the CCI, etc., may be implemented within one or more applicationspecific integrated circuits (ASICs), digital signal processors (DSPs),digital signal processing devices (DSPDs), programmable logic devices(PLDs), field programmable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, electronic devices, other electronicunits designed to perform the functions described herein, a computer, ora combination thereof.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples described herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

Those of ordinary skill in the art would understand that information andsignals may be represented using any of a variety of differenttechnologies and techniques. For example, data, instructions, commands,information, signals, bits, symbols, and chips that may be referencedthroughout the above description may be represented by voltages,currents, electromagnetic waves, magnetic fields or particles, opticalfields or particles, or any combination thereof.

Those of ordinary skill would further appreciate that the variousillustrative logical blocks, modules, circuits, and algorithm stepsdescribed in connection with the embodiments disclosed herein may beimplemented as electronic hardware, computer software, or combinationsof both.

To clearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a Digital SignalProcessor (DSP), an Application Specific Integrated Circuit (ASIC), aField Programmable Gate Array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration. The steps of a method or algorithm described inconnection with the embodiments disclosed herein may be embodieddirectly in hardware, in a software module executed by a processor, orin a combination of the two. A software module may reside in RandomAccess Memory (RAM), flash memory, Read Only Memory (ROM), ElectricallyProgrammable ROM (EPROM), Electrically Erasable Programmable ROM(EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any otherform of storage medium known in the art. An exemplary storage medium iscoupled to the processor such that the processor can read informationfrom, and write information to, the storage medium. In the alternative,the storage medium may be integral to the processor. The processor andthe storage medium may reside in an ASIC. The ASIC may reside in a userterminal In the alternative, the processor and the storage medium mayreside as discrete components in a user terminal Therefore, the presentinvention is not to be limited except in accordance with the followingclaims.

The invention claimed is:
 1. A transmitting apparatus for transmitting asignal within a predetermined spectral mask defining a range of powerdensity limits across a frequency range, the apparatus comprising: afirst signal source for providing a first input signal; a second signalsource for providing a second input signal; a filter coupled to thefirst signal source for low-pass filtering the first input signal toproduce a first filtered signal and coupled to the second signal sourcefor low-pass filtering the second input signal to produce a secondfiltered signal, the filter being selectively operable in a first modein which an input signal is filtered within a narrower bandwidth and ina second mode in which an input signal is filtered within a broaderbandwidth, wherein the first signal is filtered according to the firstmode and the second signal is filtered according to the second mode; amodulator coupled to the filter for modulating the first filtered signalto produce a first modulated signal and the second filtered signal toproduce a second modulated signal; a transmitter, coupled to themodulator, for transmitting the first modulated signal at a requiredpower level as a transmitted signal having a spectral envelope and fortransmitting the second modulated signal, wherein the first modulatedsignal and the second modulated signal are transmitted substantiallysimultaneously; and a controller for controlling operation of the filterdepending on the required power level such that the first input signalis so filtered in the first mode, when the required power level ishigher, that the spectral envelope of the transmitted signal is withinthe spectral mask, and is so filtered in the second mode, when therequired power level is lower, that the spectral envelope of thetransmitted signal remains within the spectral mask.
 2. The transmittingapparatus as claimed in claim 1, wherein the controller is arranged tocontrol the filtering in the second mode so that the spectral envelopeof the transmitted signal extends across substantially the whole of thefrequency range of the spectral mask.
 3. The transmitting apparatus asclaimed in claim 1, wherein the transmitter is arranged to transmit amodulated signal at a maximum power level when the filter is operatingin the first mode.
 4. The transmitting apparatus as claimed in claim 1,comprising a combiner for combining the first and second modulatedsignals.
 5. The transmitting apparatus as claimed in claim 1, comprisinga phase shifter for phase shifting at least one of the first and secondinput signals by a phase shift to produce first and second phase-shiftedsignals, and wherein the first and second phase-shifted signals arecoupled to the modulator.
 6. The transmitting apparatus as claimed inclaim 5, wherein the phase shifter is configured to perform the phaseshifting so that the first and second phase shifted signals are offsetfrom each other by a phase offset.
 7. The transmitting apparatusaccording to claim 5, wherein the first and second input signals eachcomprise a sequence of symbols, and the phase shifter is operable toincrement the phase shift for each symbol.
 8. The transmitting apparatusas claimed in claim 1, comprising a phase shifter for phase shifting atleast one of the first and second filtered signals by a phase shift toproduce first and second phase-shifted signals, and wherein the firstand second phase-shifted signals are coupled to the modulator.
 9. Thetransmitting apparatus as claimed in claim 8, wherein the phase shifteris configured to perform the phase shifting so that the first and secondphase shifted signals are offset from each other by a phase offset. 10.The transmitting apparatus according to claim 8, wherein the first andsecond input signals each comprise a sequence of symbols, and the phaseshifter is operable to increment the phase shift for each symbol.
 11. Amethod of transmitting a signal within a predetermined spectral maskdefining a range of power density limits across a frequency range, themethod comprising: providing a first input signal; providing a secondinput signal; low-pass filtering the first input signal to produce afirst filtered signal and low-pass filtering the second input signal toproduce a second filtered signal, the filtering being performed in oneof a first mode and a second mode, in which first mode an input signalis filtered within a narrower bandwidth and in which second mode aninput signal is filtered within a broader bandwidth, wherein the firstsignal is filtered according to the first mode and the second signal isfiltered according to the second mode; modulating the first filteredsignal to produce a first modulated signal and modulating the secondfiltered signal to produce a second modulated signal; transmitting thefirst modulated signal at a required power level as a transmitted signalhaving a spectral envelope and transmitting the second modulated signal,wherein the first modulated signal and the second modulated signal aretransmitted substantially simultaneously; and controlling the filteringdepending on the required power level such that the first input signalis so filtered in the first mode, when the required power level ishigher, that the spectral envelope of the transmitted signal is withinthe spectral mask, and is so filtered in the second mode, when therequired power level is lower, that the spectral envelope of thetransmitted signal remains within the spectral mask.
 12. The method asclaimed in claim 11, comprising controlling the filtering in the secondmode so that the spectral envelope of the transmitted signal extendsacross substantially the whole of the frequency range of the spectralmask.
 13. The method as claimed in claim 11, wherein in the first modethe required power level is a maximum power level.
 14. The method asclaimed in claim 11, comprising combining the first and second modulatedsignals.
 15. The method as claimed in claim 11, comprising phaseshifting at least one of the first and second input signals by a phaseshift to produce first and second phase-shifted signals, and modulatingthe first and second phase-shifted signals.
 16. The method as claimed inclaim 15, comprising performing the phase shifting so that the first andsecond phase shifted signals are offset from each other by a phaseoffset.
 17. The method according to claim 15, wherein the first andsecond input signals each comprise a sequence of symbols, and the methodcomprises incrementing the phase shift for each symbol.
 18. The methodas claimed in claim 11, comprising phase shifting at least one of thefirst and second filtered signals by a phase shift to produce first andsecond phase-shifted signals, and modulating the first and secondphase-shifted signals.
 19. The method as claimed in claim 18, comprisingperforming the phase shifting so that the first and second phase shiftedsignals are offset from each other by a phase offset.
 20. The methodaccording to claim 18, wherein the first and second input signals eachcomprise a sequence of symbols, and the method comprises incrementingthe phase shift for each symbol.
 21. A transmitting apparatus fortransmitting a signal within a predetermined spectral mask defining arange of power density limits across a frequency range, the apparatuscomprising: means for providing a first input signal; means forproviding a second input signal; means for filtering the first inputsignal to produce a first filtered signal and for filtering the secondinput signal to produce a second filtered signal, the means forfiltering being selectively operable, in a first mode, to filter aninput signal within a narrower bandwidth and, in a second mode, tofilter an input signal within a broader bandwidth, wherein the firstsignal is filtered according to the first mode and the second signal isfiltered according to the second mode; means for modulating the firstfiltered signal to produce a first modulated signal and for modulatingthe second filtered signal to produce a second modulated signal; meansfor transmitting the first modulated signal at a required power level asa transmitted signal having a spectral envelope and for transmitting thesecond modulated signal, wherein the first modulated signal and thesecond modulated signal are transmitted substantially simultaneously;and means for controlling operation of the filter depending on therequired power level such that the input signal is so filtered in thefirst mod; when the required power level is higher, that the spectralenvelope of the transmitted signal is within the spectral mask, and isso filtered in the second mode, when the required power level is lower,that the spectral envelope of the transmitted signal remains within thespectral mask.
 22. The transmitting apparatus as claimed in claim 21,further comprising means for controlling the filtering in the secondmode so that the spectral envelope of the transmitted signal extendsacross substantially the whole of the frequency range of the spectralmask.
 23. The transmitting apparatus as claimed in claim 21, furthercomprising means for transmitting the first modulated signal at amaximum power level when the means for filtering is operating in thefirst mode.
 24. The transmitting apparatus as claimed in claim 21,comprising combining means for combining the first and second modulatedsignals.
 25. The transmitting apparatus as claimed in claim 21,comprising means for phase shifting at least one of the first and secondinput signals by a phase shift to produce first and second phase-shiftedsignals, and wherein the first and second phase-shifted signals arecoupled to the means for modulating.
 26. The transmitting apparatus asclaimed in claim 25, wherein the means for phase shifting is configuredto perform the phase shifting so that the first and second phase shiftedsignals are offset from each other by a phase offset.
 27. Thetransmitting apparatus as claimed in claim 25, wherein the first andsecond signals each comprise a sequence of symbols, and the means forphase shifting is operable to increment the phase shift for each symbol.28. The transmitting apparatus as claimed in claim 21, comprising meansfor phase shifting at least one of the first and second filtered signalsby a phase shift to produce first and second phase-shifted signals, andwherein the first and second phase-shifted signals are coupled to themeans for modulating.
 29. The transmitting apparatus as claimed in claim28, wherein the means for phase shifting is configured to perform thephase shifting so that the first and second phase shifted signals areoffset from each other by a phase offset.
 30. The transmitting apparatusas claimed in claim 28, wherein the first and second signals eachcomprise a sequence of symbols, and the means for phase shifting isoperable to increment the phase shift for each symbol.
 31. Atransmitting apparatus for transmitting a signal within a predeterminedspectral mask defining a range of power density limits across afrequency range, the apparatus comprising: a processor; memory inelectronic communication with the processor; and instructions stored inmemory, the instructions being executable by the processor to: provide afirst input signal; provide a second input signal; low-pass filter thefirst input signal to produce a first filtered signal and low-passfilter the second input signal to produce a second filtered signal, thefiltering being performed in one of a first mode and a second mode, inwhich first mode an input signal is filtered within a narrower bandwidthand in which second mode an input signal is filtered within a broaderbandwidth; modulate the first filtered signal to produce a firstmodulated signal and modulate the second filtered signal to produce asecond modulated signal; transmit the first modulated signal at arequired power level as a transmitted signal having a spectral envelopeand transmit the second modulated signal, wherein the first modulatedsignal and the second modulated signal are transmitted substantiallysimultaneously; and control the filtering depending on the requiredpower level such that the first input signal is so filtered in the firstmod; when the required power level is higher, that the spectral envelopeof the transmitted signal is within the spectral mask, and is sofiltered in the second mode, when the required power level is lower,that the spectral envelope of the transmitted signal remains within thespectral mask.
 32. The apparatus as claimed in claim 31, wherein theinstructions are executable by the processor to control the filtering inthe second mode so that the spectral envelope of the transmitted signalextends across substantially the whole of the frequency range of thespectral mask.
 33. The apparatus as claimed in claim 31, wherein theinstructions are executable by the processor, in the first mode, tocontrol the filtering based on the required power level when therequired power level is a maximum power level.
 34. The apparatus asclaimed in claim 31, wherein the instructions are executable by theprocessor to combine the first and second modulated signals.
 35. Theapparatus as claimed in claim 31, wherein the instructions areexecutable by the processor to phase shift at least one of the first andsecond input signals by a phase shift to produce first and secondphase-shifted signals, and to modulate the first and secondphase-shifted signals.
 36. The apparatus as claimed in claim 35, whereinthe instructions are executable by the processor to perform the phaseshifting so that the first and second phase shifted signals are offsetfrom each other by a phase offset.
 37. The apparatus as claimed in claim35, wherein the instructions are executable by the processor to performthe phase shifting when the first and second input signals each comprisea sequence of symbols, by incrementing the phase shift for each symbol.38. The apparatus as claimed in claim 31, wherein the instructions areexecutable by the processor to phase shift at least one of the first andsecond filtered signals by a phase shift to produce first and secondphase-shifted signals, and to modulate the first and secondphase-shifted signals.
 39. The apparatus as claimed in claim 38, whereinthe instructions are executable by the processor to perform the phaseshifting so that the first and second phase shifted signals are offsetfrom each other by a phase offset.
 40. The apparatus as claimed in claim38, wherein the instructions are executable by the processor to performthe phase shifting when the first and second input signals each comprisea sequence of symbols, by incrementing the phase shift for each symbol.41. A computer program product, comprising: non-transitory computerreadable medium comprising: code for causing a computer to transmit asignal within a predetermined spectral mask defining a range of powerdensity limits across a frequency range, the code comprisinginstructions to: provide a first input signal; provide a second inputsignal; low-pass filter the input signal to produce a first filteredsignal and low-pass filter the second input signal to produce a secondfiltered signal, the filtering being performed in one of a first mode inwhich an input signal is filtered within a narrower bandwidth and asecond mode in which an input signal is filtered within a broaderbandwidth, wherein the first signal is filtered according to the firstmode and the second signal is filtered according to the second mode;modulate the first filtered signal to produce a first modulated signaland modulate the second filtered signal to produce a second modulatedsignal; transmit the first modulated signal at a required power level asa transmitted signal having a spectral envelope and transmit the secondmodulated signal, wherein the first modulated signal and the secondmodulated signal are transmitted substantially simultaneously; andcontrol the filtering depending on the required power level such thatthe first input signal is so filtered in the first mod; when therequired power level is higher, that the spectral envelope of thetransmitted signal is within the spectral mask, and is so filtered inthe second mode, when the required power level is lower, that thespectral envelope of the transmitted signal remains within the spectralmask.